A new margin rule for the largest players on the futures exchange netted $1.5 billion when it went into force after trading closed on Tuesday. A source close to the exchange said yesterday that the revised fees, which increased the margin by 50 per cent for investors holding more than 10,000 contracts, were encouraging those with significant holdings to pare back their positions. Open interest in September fell to 96,242 contracts at the close of trading on Tuesday - the latest figure available - compared with 103,074 open contracts on Monday. Officials have said that should the rule fail to reduce the distribution of contracts to what the futures exchange calls more normal levels, an absolute limit of 10,000 contracts may be introduced. For an investor who held 10,001 contracts, the margin call would have risen to about $1.2 billion from about $800 million.