Human T-lymphocytes have been treated with benzo[a]pyrene diolepoxide (BPDE) in vitro and T-cell clones mutated in the hprt gene have been isolated. The mutant frequencies in BPDE-treated T-cell cultures were on average 24-fold higher than those of untreated cultures. Thus, BPDE is a potent inducer of gene mutation in this system. In order to examine which types of mutations are induced by BPDE in human cells, 41 spontaneous and 44 BPDE-induced mutant clones have been characterized using the Southern blot technique. In addition, rearrangements of the T-cell-receptor beta and gamma loci have been used to determine the proportion of isolated clones that are unique, and thus likely to represent independent mutational events. Out of 23 independent spontaneous mutants 4 had large hprt alterations that could be detected on Southern blots. Two of these alterations, deletions of exons 2-6, have been confirmed using PCR of hprt cDNA and direct sequencing of the PCR product. All 33 independent BPDE-induced mutants had normal hprt restriction patterns which indicates that BPDE is mainly a point mutagen in this system.