The relationships between replicative DNA synthesis and retinoic acid (RA)-induced differentiation of human promyelocytic leukaemic (HL-60) cells are evaluated with the use of Aphidicolin, a specific and reversible inhibitor of DNA polymerase alpha (alpha). Addition of a sublethal concentration of Aphidicolin (0.4 microM) in culture for 3 days suppresses DNA synthesis to a similar level of the resting stage (day 8) in control cultures. DNA synthesis is reactivated to the level observed in the growing stage of control cultures once Aphidicolin is removed after 3 days in culture. The level of DNA synthesis at the early stage of RA-induction (day 3) is suppressed by only 17% when compared to control cultures. The inhibitory effect of Aphidicolin on DNA synthesis in both control cultures and RA-induced cell cultures is similar. However, no reactivation of DNA synthesis is observed after removal of Aphidicolin on day 3 from RA-induced cell cultures. Flow cytometric analysis of DNA content on day 3 reveals that cells accumulate in G1 and early S phases of the cell cycle after exposure to Aphidicolin with or without RA. Of interest is the fact that, while Aphidicolin alone did not induce cells to differentiate, neither did it interfere with RA-induced cell differentiation (the rate of RA-induced cell differentiation in the presence of Aphidicolin is similar to that of RA-treated cultures in the absence of Aphidicolin). These results suggest that the combined use of Aphidicolin and RA may inhibit leukaemic cell proliferation more effectively without causing severe cytotoxicity and without interfering with RA-induced cell differentiation.