The asthma drug clenbuterol has been detected in pigs from 17 local and two mainland farms.
The Agriculture and Fisheries Department said yesterday 417 urine samples had been collected from pigs sent for slaughter, 34 of which were found to contain clenbuterol.
The samples were collected under a monitoring system implemented last Saturday.
'The department has successfully identified the farms of origin by tattoo marks, and follow-up raids to local farms were jointly conducted with the Department of Health in the past few days,' a spokesman said.
Samples were collected for testing by the government chemist and suspected offenders would be prosecuted.
The department had liaised with its mainland counterpart to temporarily suspend two mainland farms from supplying Hong Kong and to conduct follow-up investigations.
Clenbuterol has been used in animal feed, but is banned as a health safeguard.
Hong Kong farmers yesterday claimed pigs smuggled from the mainland were the source of the contaminated offal. Local pig farmers' representative Wong Kwong-wing said: 'I heard some farmers bought smuggled pigs and mixed them with locally reared ones.
'The contaminated pigs can't be from local farms. They must have been smuggled from the mainland where drug management is far more backward than ours.' At least 17 people reportedly fell ill after eating offal from pigs fed with the drug, prompting a ban on sales of offal in May.