The genetical analysis, by protoplast fusion, of the sterile strain ED22 of Schizosaccharomyces pombe is described. Two major mutations are harboured by this strain. One, cdc 25.22, is conditionally defective in mitosis. The other mutation, ste 1.1, causes sterility in strains of h-, h+ or mat 2.102 mating-type. Sterility is due to the failure of cell agglutination. We present evidence that ste 1.1 is defective in the production of a non-diffusible and non-mating-type specific factor. ste 1 and cdc 25 both map on chromosome I and are loosely linked.