The in vitro effects of the recreational drugs, ethanol (EtOH) and nicotine, on natural killer (NK) antibody-dependent cellular cytotoxic (ADCC) and lymphokine-activated killer (LAK) cell activities on normal lymphocytes were investigated. Lymphocytes precultured with EtOH at concentrations of 0.4 and 0.6% (v/v) produced significant suppression of NK and ADCC activities. In target-binding assays, EtOH decreased the target-binding capacity of effector cells. EtOH also inhibited the activities of Percoll-separated, NK-enriched large granular lymphocytes. EtOH-induced inhibition of NK activity could be reversed by incubating lymphocytes for 1 hr with interferon. The generation and lytic capacity of LAK cells was also significantly depressed by EtOH when added at the initiation of culture. Nicotine at concentrations of 5 and 10 micrograms/ml, when added directly to mixtures of effector and target cells, produced significant inhibition of NK activity. Nicotine (2 micrograms/ml) and EtOH (0.01, 0.1, and 0.2%) at noninhibitory concentrations when added separately, showed significant suppression of NK activity when used in combination. Pretreatment of target cells with either EtOH or nicotine for 4 hr did not affect cytotoxic activity. Inhibition of cytotoxicity was also not due to direct toxicity of effector cells because lymphocytes treated with either EtOH or nicotine showed normal 51Cr release and their viability was comparable to that of untreated control cells. These studies demonstrate that EtOH and nicotine have significant immunomodulatory effects on the cytotoxic activities of human lymphocytes which may be of clinical relevance.