REGIONAL Services Department staff seized samples of white cabbage from a Tai Po market stall yesterday after four people became ill after eating the vegetable. A Government spokesman said doctors at Queen Elizabeth Hospital had confirmed suspected food poisoning after treating a family of four living in Tai Wo Estate, Tai Po. ''The Department of Health has informed the Regional Services Department to take action by sending men to collect vegetable samples from the hawker stall,'' he said. ''The samples will be tested in the laboratory to see if they have been contaminated. We are still investigating the source of the white cabbage concerned.'' Three of the four members of the family, Mr Tsang Tam-tim, 53, his children, 13-year-old son Kin-chi and 12-year-old daughter Yin-hung, were treated and discharged. The mother who bought the cabbage yesterday morning, Mrs Tsang Wan-wah, 45, was detained for observation. The children later told their grandmother they felt uncomfortable, probably due to food poisoning.