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D‐Penicillamine, an amino acid analogue of cysteine, has been shown to inhibit the transactivation of HIV‐1 LTR by the transactivator protein, tat protein. The transactivation was studied in Jurkat cells co‐transfected with plasmids containing HIV‐LTR sequences fused to the bacterial chloramphenicol acetyltransferase (CAT) gene and HIV tat gene. The expression of CAT activity was a measure of transactivation of LTR by the tat protein. Incubation of transfected Jurkat cells with D‐penicillamine led to inhibition of CAT activity. This inhibition was found to be concentration‐dependent; more than 90% inhibition of chloramphenicol acetylation was seen in extracts prepared from cultures incubated with 40 μg/ml of D‐penicillamine. Earlier experiments have shown that D‐penicillamine at 40 μg/ml can completely inhibit HIV‐1 (HTLV‐III B) replication in H9 cells [(1986) Drug Res. 36, 184–186]. These results suggest that inhibition of transactivation may be the molecular mechanism involved in the inhibition of HIV‐1 replication by D‐penicillamine.