The relationship between the structure and activity of aclacinomycin-A (ACM) metabolites was investigated in vitro in Friend leukaemia cells (FLC). The cytotoxic effect was related to the ease with which ACM and its metabolites accumulate in the nucleus. Cellular uptake and nuclear incorporation are influenced by the hexopyranoses linked to aklavinone (AKV) and by the two methyls linked to the L-rhodosamine amino groups. The effect of ACM and its metabolites on macromolecular synthesis depended on the drug concentrations and the exposure time. ACM was the most active in the inhibition of nucleic acid synthesis whereas it had no direct effect on protein synthesis even at high drug concentrations. When cells were treated for a short time with low drug concentrations (1 microM), RNA synthesis was inhibited to a greater extent than DNA synthesis. But when incubated for longer periods, inhibition of DNA synthesis increased further. RNA and DNA syntheses were both inhibited to about the same extent only when cells were exposed to the higher drug concentrations (10 microM). We conclude therefore that at low drug concentrations the effect on DNA synthesis is probably a consequence of RNA synthesis inhibition. The early DNA synthesis inhibition which occurs at higher drug concentrations may result from the direct action on the cellular genome.