THE stock exchange may draw up guidelines on so-called ''cash shell'' companies, which are listed but consist mainly of cash, after two companies announced plans to sell most of their activities. Lucky Man Properties said yesterday it was negotiating to sell its main assets, property in Repulse Bay Road, and would try to find new investments to allow it to continue to be listed. World Trade Centre Group announced at the weekend plans to sell its main asset, the World Trade Centre building in Causeway Bay, to Sun Hung Kai Properties. It has been told by the head of the exchange's listing division, Mr Herbert Hui Ho-ming, that it must give a detailed breakdown of its on-going business and investment plans when it presented the sale proposals to shareholders. ''We are concerned when a company consists principally of cash,'' said Mr Hui. But he said the level at which action would be necessary was a matter of judgement which could need formalising. Any company with cash and no business was ''directionless'', he said. ''The cash could disappear,'' Mr Hui said. ''It is the concept of the Hongkong stock exchange that every company has a business to follow.'' He agreed that it was difficult to decide when a company's cash reserves became large enough to create a cash shell. ''Guidelines are something we may think about, but the concept of a cash shell is a very judgemental one.'' Statements by the two companies in question suggested Lucky Man was being pressed hardest to explain its position, although both were determined to retain listed status, and were looking for uses for their cash. A financial adviser to Lucky Man said yesterday that the company would use its cash to invest in property in Hongkong and China, and said if a deal was going to be agreed upon then it should take no more than a week to negotiate. At Lucky Man, it was not clear whether the company had any activities apart from rental of the properties which may be sold, which the company's announcement described as its ''major asset''. None were mentioned in the documents sent to shareholders which helped put the company back on to the exchange after a four-year suspension, although shareholders were given brief details of plans to diversify into property or trade in building materialsin China. Mr David Lee, general manager of World Trade Centre Group, yesterday refused to give his company's financial position if the building sale went ahead. It would receive net cash of $1.57 billion from Sun Hung Kai Properties. Mr Hui said he had been told that after the sale the company ''will have a substantial amount of cash, but it is not principally cash''. A dispensation has been made for investment companies, which may be permitted to hold much of their assets in cash or short-dated securities. Lucky Man's share price has been consistently many times its net asset value, apparently pulled up by speculators who believed it would be used as a shell company by a mainland concern. The prospectus that supported Lucky Man's return to the exchange put net asset value at $1.31, against the closing share price yesterday of $8. The announcement by the company that it was selling its main asset had little effect on the share price, which rose five cents. The company returned to the exchange last October after a drama involving missing share certificates, board changes and litigation. Lucky Man owns both towers of the Fairview Terrace development. The second was bought as part of the re-listing, a move the company said would increase its financial strength and assist the re-listing process.