Wild-type p53 causes cell-cycle arrest at late G1 phase and induction of apoptosis by up- regulation of waf1 and bax, respectively. Although in many cancer cells p53 is frequently mutated and loses its functions, we have proposed that mutant p53 may be involved in the anticancer mechanism of 9-hydroxy ellipticine (9HE). It was shown using flow cytometry that 9HE (10 microM) caused induction of apoptosis in G1 phase of the cell cycle in mutant p53 (p53ala143, p53his175, orp53his273) transfected Saos-2 cells, but not in p53-deficient parental Saos-2 cells. Similar induction of apoptosis was observed 24-48 h after treatment with 9HE in mutant p53-containing SW480, SK-BR-3 and MKN-1, but not in p53-deficient KATO m cells. Using G1 phase cells isolated by centrifugal elutriation, it was confirmed that 9HE caused cell cycle arrest at G1 phase and subsequently induced G1 phase-restricted apoptosis. In accordance with the induction of arrest and apoptosis in G1 phase, 9HE caused up-regulation of waf1 and bax mRNA in mutant p53-containing cells, but not in p53-deficient parental Saos-2 cells. In control experiments, adriamycin (ADR) showed neither G1-restricted apoptosis nor elevation of bax mRNA. It is suggested that 9HE may cause G1 arrest and induction of G1 phase-restricted apoptosis by restoring the wild-type function of mutant p53 protein.