We have screened expression libraries for mammalian cDNAs capable of suppressing defects in ras1- Schizosaccharomyces pombe. Both the RAP1A and RAP1B genes were identified in this manner. They suppress defects in cell morphology and sporulation, although not conjugation. In contrast, RAP genes do not suppress phenotypes in the yeast Saccharomyces cerevisiae that are deficient in RAS. Indeed, expression of RAP1A appears to antagonize the activated S. cerevisiae RAS2val19 gene. These results indicate that RAP proteins can interact with RAS targets, sometimes productively, sometimes nonproductively.