Two independent lines of CHO heterozygous for adenine phosphoribosyltransferase (aprt) have been isolated after mutagenesis by sib selection on the basis of partial resistance to diaminopurine (DAP). The mutants were stable and contained 35-50% of the wild-type specific activity of APRT, as did partially DP-resistant mutants isolated directly from nonmutagenized cultures by us as well as by others. However, the mutation rate to the homozygous aprt-/- state (full DAP resistance) in cultures of the sib-selected cells was generally 10- to 100-fold higher than in cultures of heterozygotes isolated directly. A further unusual property was that this high mutation rate varied greatly from subclone to subclone, by as much as a factor of 10(3). Mutation at this allele of aprt therefore did not occur randomly in these cells. Mutagenesis did not significantly increase this high mutation frequency but had a great effect on the mutation frequency of the directly isolated heterozygotes. It is proposed that mutation at the aprt locus occurs by two different mechanisms, but the high frequency mechanism can be responsible for mutation of only one of the two alleles.