Nine independent mutants which are supersensitive (ssl-) to G1 arrest by the mating hormone a-factor were isolated by screening mutagenized Saccharomyces cerevisiae MAT alpha cells on solid medium for increased growth inhibition with a-factor. These mutants carried lesions in two complementation groups, ssl1 and ssl2. Mutations at the ssl1 locus were mating type specific: MAT alpha ssl1- cells were supersensitive to a-factor but MATa ssl1- were not supersensitive to alpha-factor. In contrast, mutations at the ssl2 locus conferred supersensitivity to the mating hormone of the opposite mating type on both MAT alpha and MATa cells. The alpha-cell specific capacity to inactivate externally added a-factor was shown to be lacking in MAT alpha ssl1- mutants whereas MAT alpha ssl2- cells were able to inactivate a-factor. Complementation analysis showed that ssl2 and sst2, a mutation originally isolated as conferring supersensitivity to alpha-factor to MATa cells, are lesions in the same gene. The ssl1 gene was mapped 30.5 centiMorgans distal to ilv5 on chromosome XII.