The effects of interview mode on 18- to 34-year-olds' self-reported tobacco, alcohol, marijuna, and cocaine use were investigated. RDD telephone and personal drug use surveys of the state of New Jersey were conducted in 1986–87. In the personal interview, respondents recorded their drug use on self-administrated answer sheets. Compared to the area probability sample in the personal interview, RDD yielded a sample of blacks higher in income and education, and more likely to be married and currently employed; white SES was also higher in the telephone sample, but to a lesser degree than for blacks. Controlling for demographic characteristics and RDD's exclusion of non-telephone households, the telephone survey yielded significantly lower estimates of blacks' alcohol consumption, and lifetime and recent marijuana use. Whites' alcohol consumption was slightly lower by telephone; otherwise, estimates of whites' use of the four substances were nearly identical in the two modes. Sample coverage, respondent demographic characteristics, and racial matching of interviewer and respondent did not account for the significant mode differences. Characteristics of the interview situation itself, such as provision of privacy in the self-administered format, may have influenced tendencies toward socially desirable responding to a threatening topic such as drug use.