Mutations in six genes that eliminate responsiveness of Saccharomyces cerevisiae a cells to alpha-factor were examined by assaying the binding of radioactively labeled alpha-factor to determine whether their lack of responsiveness was due to the absence of alpha-factor receptors. The ste2 mutants, known to be defective in the structural gene for the receptor, were found to lack receptors when grown at the restrictive temperature; these mutations probably affect the assembly of active receptors. Mutations in STE12 known to block STE2 mRNA accumulation also resulted in an absence of receptors. Mutations in STE4, 5, 7, and 11 partially reduced the number of binding sites, but this reduction was not sufficient to explain the loss of responsiveness; the products of these genes appear to affect postreceptor steps of the response pathway. As a second method of distinguishing the roles of the various STE genes, we examined the sterile mutants for suppression. Mating of the ste2-3 mutant was apparently limited by its sensitivity to alpha-factor, as its sterility was suppressed by mutation sst2-1, which leads to enhanced alpha-factor sensitivity. Sterility resulting from each of four ste4 mutations was suppressed partially by mutation sst2-1 or by mutation bar1-1 when one of three other mutations (ros1-1, ros2-1, or ros3-1) was also present. Sterility of the ste5-3 mutant was suppressed by mutation ros1-1 but not by sst2-1. The ste7, 11, and 12 mutations were not suppressed by ros1 or sst2. Our working model is that STE genes control the response to alpha-factor at two distinct steps. Defects at one step (requiring the STE2 gene are suppressed (directly or indirectly) by mutation sst2-1, whereas defects at the other step (requiring the STE5 gene) are suppressed by the ros1-1 mutation. The ste4 mutants are defective for both steps. Mutation ros1-1 was found to be allelic to cdc39-1. Map positions for genes STE2, STE12, ROS3, and FUR1 were determined.