The anti-HIV activity of an antiseptic miramistin in the presence of fetal calf serum (FCS) was studied. It has been recently shown that cation detergent miramistin at a concentration of 100 micrograms/ml prevented HIV-1 replication in MT-4 cells when these cells were cocultivated with MT-4 cells previously infected and treated with the detergent in a protein-free medium. Under such conditions 50 micrograms of miramistin per ml delayed HIV propagation in MT-4 cells by 14 days. Detergent-dependent arrest of HIV development was abolished by addition of FCS at a final concentration of 50%. Miramistin in a dose of 100 micrograms/ml prevents HIV-1 replication in Jurkat-tat cells when they are cocultivated with the cells of the same line previously infected and treated by the detergent in the presence of 5% FCS. FCS in concentrations from 12.5 to 75% prevents the anti-HIV activity of miramistin in a dose of 100 micrograms/ml.