HE sex-linked lethal test provides an objective, quantitative approach to Tmutagenesis in Drosophila. For radiation mutagenesis the analysis is usually limited to a single generation, the F, progeny of irradiated PI males or females. This procedure was established by MULLER (1928), who demonstrated that the bulk of induced mutations were present in these F, cultures. The sampling of single females from each F, culture revealed no important “sorting out” of altered components of the originally treated paternal X chromosomes in the F, generation. Similarly, the frequency of sex-linked lethals in the F, generation was shown to be insignificant in comparison with that obtained in the F, generation. In MULLER’S experiments the sampling of one female from each culture for each generation enabled him to calculate the total mutation frequency with maximum accuracy. This method of sampling, however, does not provide a measure of the distribution of mutant and nonmutant tissue in the gonads of each generation tested. For chemical mutagenesis, furthermore, the distribution of sex-linked lethal mutations to the gonads is probably different from that found for radiation studies, where the mechanism of mutagenesis produces chiefly complete mutations. AUERBACH ( 1945) demonstrated a high degree of mosaicism for visible mutations with chemical mutagens. Following this, investigations comparing the ratio of lethal to visible mutations revealed a striking difference for various chemical mutagens suggesting a differential mutagenic effect on the classes of mutations themselves (FAHMY and FAHMY 1955). This was reinterpreted by CARLSON and OSTER (1961) on the basis of their studies with the monofunctional alkylating agent, ICR-100 (the compound is identified by the institutional numbering system of H. J. CREECH at the Institute for Cancer Research, Philadelphia, Pennsylvania). CARLSON and OSTER predicted that the same mechanism of mosaicism found by AUERBACH would lead to an under-estimation of F, lethals (where the absence of the treated paternal X chromosome is the basis of the lethal classification). Since mosaic sex-linked lethals would be represented by F, female gonads containing both mutant and nonmutant paternal X chromosomes, these would be misclassified as nonlethals or semilethals in the F, generation. This was directly demonstrated by CARLSON and OSTER (1962) using ICR-100. Also, BROWNING and ALTENBURG (1961), using several chemical mutagens, demonstrated that the ratio of sex-linked lethal mutations to specific visible mutations was dependent on their capacity for inducing mosaicism. It is the purpose of this paper