A series of prototrophic fragile strains of different ploidy (2n, 3n and 4n) has been genetically constructed on the basis of haploid srb1 containing segregants of the fragile Saccharomyces cerevisiae mutant VY 1160. The strains have been characterized by several criteria. In regard to generation time, biomass yield, and nucleic acids content of the cells, the tetraploid srb1 homozygous hybrid is indistinguishable from an industrial strain of S. cerevisiae. However, it is characterized by a higher protein content. Unlike any other laboratory or industrial strains, the original mutant and these hybrids possess an ability for lysis upon suspension in hypotonic solutions. The dependence of the percentage of lysed cells on the growth phase and concentration of osmotic stabilizer in the medium has been investigated. The quantity of proteins in the soluble and insoluble fractions obtained after lysis of these strains by osmotic shock has been determined. These hybrids can be considered as a potential industrial source of proteins for nutritional purposes.