The p53 tumor suppressor gene has been implicated in the induction of apoptosis in several cell systems. We have recently reported than transiently-transfected wt p53 is capable of inducing apoptosis in certain transformed cell lines. We demonstrated by quantitative analysis using flow cytometry that apoptosis was restricted to the population expressing wt, but not mutant, p53. In the present study we use this model system to analyse the functional domain of p53 in the induction of apoptosis. Several constructs expressing mutations or deletions in the C-terminal oligomerization domain, the N-terminal transactivation domain or the central DNA-binding domain were introduced into HeLa cells, and the ability of the expressed proteins to induce apoptosis was evaluated. All the functional domains were found to be necessary for the induction of apoptosis. In addition, cycloheximide and actinomycin D inhibited wt p53-induced apoptosis. We therefore conclude that p53 acts in this cell system, at least in part, as a transcriptional activator in the induction of apoptosis.