Fifty-two cultures grown under standard conditions from separate subclones of haploid Saccharomyces strain X2180-1A were exposed to standard treatment with nitrous acid (NA). When the duration of NA-treatment was the same:a) cell survival varied markedly and the frequency distribution of its values differed significantly from the normal one; b) the overall frequency of induced ade1- and ade2-mutants (OMF) and the proportion of complete mutants among them (PCM) also displayed significant subclonal variation, apparently co-ordinated with the variation in survival. In the same experiments several different types of dependence of the OMF and PCM on the duration of NA-treatment were found. Thus the characteristics of the action of NA on yeast cells appeared to be unstable. Subclones with pronounced and reproducible differences in sensitivity to the lethal action of NA were isolated from X2180-1A without any induction or enrichment technique. This fact proves the genetic nature of the instability discovered, i.e. this instability appears to reflect the behavior of unstable genetic factors playing, along with the stable ones, a role in the control of NA-sensitivity of X2180-1A cells.