Summary It is shown that the lethal action of nitrogen mustard on spores of Aspergillus niger can be described in terms of the target theory, if one assumes that the intensity of this action varies with time. A two-hit curve can be fitted on the experimental data, on the assumption that a decrease of lethality depends on the inactivation of the substance with time. By diluting the suspension containing the spores with a solution of glycine and sodium bicarbonate, the action of nitrogen mustard is stopped. The lethal action of nitrogen mustard is much weaker at 0° C than at 27° C. Even though exact mutation rates have not been evaluated, an outstanding number of morphological and physiological mutants among the colonies developing from irradiated spores have been observed.