Mutagenesis studies have indicated that the deamination of cytosine in UV-induced cyclobutane pyrimidine dimers is a key part of an error-free process that can account for most of the C-->T base specificity that frequently characterizes UV-induced mutagenesis. The kinetics of deamination, as inferred from the mutagenic effect of delayed photoreactivation, is remarkable in its resemblance to a step function. To study the kinetics from a different point of view, we used an enzymatic approach combining photolyase and uracil-N-glycosylase treatment to detect the formation of uracil in UV-irradiated single or double-stranded infectious DNA of phage S13. Formation of abasic sites by removal of uracil was inferred from loss of infectivity. It is concluded that no uracil appeared at 37 degrees C within 20 minutes (single-stranded DNA) or 40 minutes (double-stranded DNA) after irradiation, but following those latent periods, most of the uracil residues rapidly appeared within a brief 14 minute period centered at 29 minutes (single-stranded DNA) and 55 minutes (double-stranded DNA) after irradiation. The timing appears to fully confirm the previous evidence that dimer cytosines in DNA deaminate with step-function-like kinetics. Furthermore, the appearance of uracil was shown to be dependent on the UV-induction of cyclobutane dimers containing cytosine.