In mouse testis, PP1?1 is restricted mainly to somatic cells, while PP1?2 is primarily located in secondary spermatocytes and germ cells, and the only isoform detectable in mammalian spermatozoa. Targeted deletion of the PP1? gene results in sterile male mice. Because the most prominent marker of this phenotype is aberrant spermatid morphogenesis, its primary cause may be the absence of PP1?2 since PP1?1 is mainly in somatic cells. To better understand the role of PP1?2 in mammalian testis/sperm, this dissertation was undertaken to explore PP1?2 interactions with its regulatory proteins in testis/sperm. Wild-type and mutant isoforms of PPP1R11/inhibitor 3 (I3), PPP1R2/inhibitor 2 (I2) and a novel protein similar to I2 were studied. The studies from this dissertation provide important insights into the regulation of PP1?2 in testis and sperm.