Previous work with two closely related human lymphoblast cell lines demonstrated that WTK1 cells are more resistant than TK6 cells to X-ray-induced cytotoxicity, but more mutable at both the TK and HPRT loci. It was subsequently determined that WTK1 cells overexpress a mutant form of the tumor suppressor gene p53, while TK6 cells correctly express wild-type p53. Thus these two cell lines allowed us to examine the mutational spectra at the HPRT locus in related human lymphoblast cell lines differentially expressing p53. Previously, we isolated sets of X-ray-induced and spontaneous WTK1 mutants and spontaneous TK6 mutants and analyzed the mutational spectra by the combination of multiplex polymerase chain reaction (PCR) and Southern blot analysis. Somewhat unexpectedly, we found that even though there were approximately four times as many mutants induced by X rays in WTK1 cells as in TK6 cells, there was very little difference in the mutational spectra. In the present study, to determine if there was a higher frequency of intragenic deletions among the X-ray-induced WTK1 mutants, we further examined the subsets of mutants that contained HPRT point mutations. cDNA sequence analysis was used to define the mutation precisely in 19 X-ray-induced and 25 spontaneous WTK1 mutants and 25 spontaneous TK6 mutants. While subtle differences exist in the spectra of HPRT mutations between these two cell lines, the data again suggest that p53 is associated with an increase in the frequency of mutations at HPRT without an obvious effect on the types of mutations recovered.