Cells of E. coli B/r labelled with 32P undergo inactivation during storage in liquid nitrogen (-196° C). Thawing the samples after storage and plating them immediately on complete medium yields colonies of uniform appearance. However, replica plating them on minimalmedium demonstrates, that during 32P decay there occured a significant increase in nutritionally deficient (auxotrophic) mutants. When 10-4 is the survival-rate, 0.67% of surviving cells give rise to auxotrophic colonies, 40 - 50% of which have prototrophic sectors. During storage the increase of mutants is linear with respect to inactivation of bacteria and to the fraction of 32P decayed. The experiments carried out show, that inactivation and mutation are due to the decay of 32P atoms incorporated in cellular structures and not do effects of freezing, toxicity of contaminants present in the 32P solution, or ionization of β-electrons. An increase of mutants by positive selection in the course of inactivation was excluded experimentally