MORE than 100 samples of mainland-grown vegetables have tested negative for traces of poisonous pesticides, raising concern that the current outbreak of poisoning might have been caused by gau gei grown locally. Two pesticides banned in Hong Kong, methamidophos and isocarbophos, are believed to be most likely to have caused the poisoning of 22 Hong Kong people who bought the vegetables in markets. Two remain in Queen Mary Hospital, but no new cases have been reported. The Assistant Director of the Department of Health, Dr Mahommed Bin Ali, did not rule out the possibility the vegetables could have come from Hong Kong. Testing of vegetable consignments at the border and in markets by officers from the Department of Health and the Department of Agriculture and Fisheries continued yesterday as experts tried to pinpoint the source of the outbreak. ''Today we tested several types of vegetables, including Chinese cabbage, being carried by 45 vehicles coming into Hong Kong,'' a Health Department spokesman said. ''We don't rule out the possibility that the contaminated vegetables may have originated in Hong Kong.'' Hong Kong growers contribute less than 30 per cent of the vegetables consumed locally, according to senior Agriculture and Fisheries officer Clive Lau Siu-ki. He said he believed it was unlikely the contaminated gau gei was locally grown, but the possibility could not be discounted. ''This particular crop is usually pest-free, so we can assume that the farmer was practising mixed-cropping and used pesticide on his other crop,'' he said. Another officer said there had been no perceptible drop-off in customer demand for gau gei since the outbreak. Dr Ali said about 300 trucks import vegetables into Hong Kong every day, making it almost impossible to examine every basket of vegetables. that comes across the border.