Mandatory drug testing (MDT) was introduced in all penal establishments in England and Wales by March 1996. The Home Office commissioned the Oxford Centre for Criminological Research to assess the impact of the drug testing program on the extent and nature of drug misuse. The study was based in five prisons with different functions and located in various parts of the country. The sample of prisoners interviewed reflected the range of age, gender, sentence length, and custodial settings found in the system as a whole. All of the 148 inmates interviewed had been tested under MDT in the month preceding fieldwork. The research was designed to clarify the range of responses to MDT, the reasons for any changes in the levels of drug use, and the extent to which MDT influenced inmates' choice of drugs. The study also included interviews with 146 staff and an examination of official records, including the drug-testing registers and adjudication log books. Of the 148 inmates interviewed, 37 claimed they did not use drugs in prison. More than half of the remaining 111 prisoners said that the increased risk of detection and sanctions as a result of MDT had a substantial impact on their drug abuse. Thirty of the 111 who said they had formerly taken drugs when in custody claimed to have stopped completely; 17 said they had reduced their consumption; and 7 reported altering their pattern of consumption, taking less cannabis but continuing to use heroin. Four said they had tried heroin for the first time and cut down on their cannabis use in response to MDT; none of these persisted in the use of heroin. Almost half (53) stated they had not changed their drug-taking. Prisoners seemed to be more concerned about the effect of MDT on their access to incentives and privileges than they were about being sanctioned with additional time in prison. This suggests that greater provision of privileges and incentives would be more likely than disciplinary awards alone to increase the effectiveness of MDT. 1 figure and 1 table