A new method is proposed to inhibit semiconservative DNA synthesis in cultured cells while DNA repair synthesis is being measured. The cells are treated with the DNA-crosslinking agent Trioxalen (4,5,8-trimethylpsoralen) plus near-ultraviolet light, and consequently 99.5% inhibition of replicative DNA synthesis is achieved. Additional DNA-damaging agents induce thymidine incorporation into the double-stranded regions of the DNA. The new method gave results very similar to those obtained with the benzoylated naphthoylated DEAE (BND) cellulose method using three human fibroblast strains, of which one had deficient capacity for DNA repair synthesis following treatment with γ rays and methyl methanesulfonate. The advantages of the new method are simplicity and rapidity, as well as the high extent to which replicative DNA synthesis is inhibited.