Arabinocytidine and aphicolin are inhibitors of .alpha.-DNA polymerase that affect both normal DNA replication and repair synthesis in mammalian cells. In contradiction to the prevalent hypothesis that these inhibitors merely slow the polymerization rate at incision sites near lesions, the repair synthesis resistant to inhibitors is apparently mediated by a separate pathway. Repair synthesis in contact-inhibited human cells following UV irradiation was inhibited 75-80% by arabinocytidine or aphidicolin, and most of the repair patches were not ligated into parental DNA, as judged by an enzymatic assay. The patches were not demonstrably shorter than those in untreated cells. Even following low-UV doses at which no inhibition of repair synthesis by the inhibitors was observed, a majority of the patches were not ligated. DNA polymerase .beta. is implicated in this alternate pathway, both by the known specificity of the inhibitors and by evidence from their sensitivity to S1 nuclease that the patches are not degraded in vivo and eventually become ligated into parental DNA, very slowly in the presence of inhibitors but much more rapidly following their removal. Under conditions of .alpha.-polymerase inhibition, a limited number of normal length repair patches are made, apparently by displacement synthesis, leaving displaced strands that remain substantially undegraded.