In November, 1979, a police undercover operation resulted in the arrest of 20 suspected drug dealers and ongoing investigation of an additional 20. Analysis of urine specimens of methadone clinic clients subsequent to this operation shows there was a dramatic decrease in the number of urine samples with detected opiates following the arrests. The highest number of urine samples with opiates following the drug arrests was lower than the lowest number found in samples taken before the operation. Thus, the police action resulted in fewer methadone clients using opiates and using them less often. This was true not only of those clients at the clinic that was directly linked to the arrests, but also at 2 other clinics. An examination of clinical data for 21 clients indicates that a majority showed improvement in treatment after the arrests. Results of this study document the success of police action in reducing the availability of opiates to abusers. 4 figures and 1 reference.