The effect of DNA-crosslinking agents (cisplatin and mitomycin C), a DNA-intercalating agent (adriamycin) or monofunctional psoralen (4-Met-4', 5'-dihydropsoralen plus near-ultraviolet radiation) on DNA replication in cultured mouse FM3A cells was studied by sedimentation of the pulse-labeled DNA in an alkaline sucrose gradient. There was no inhibition of the maturation process of the nascent DNA 1 h after treatment of the cells with cisplatin, mitomycin C or adriamycin. However, this process was inhibited progressively during further incubation for 4 to 16 h without the agents, and accumulation of the nascent DNA fragments was observed. In the case of DNA-crosslinking agents, this inhibition may be correlated with an increase in the number of DNA-crosslinks formed during incubation. However, direct inhibition of maturation was not likely since the number was small, up to 4.9 per 10(9) Da. After treatment of cells with monofunctional psoralen, no inhibition of maturation of the nascent DNA fragments was observed. In all cases, there was no degradation of DNA during incubation.