The perinuclear theca (PT) is a cytoskeletal structure that covers the nucleus of mammalian spermatozoa and is believed to have a membrane-binding role. The objectives of this study were to analyze the protein composition of the mouse PT, to identify and sequence its major protein component, and to characterize this protein's transcriptional and translational origins during spermatogenesis. The PT was extracted from demembranated and acrosome-depleted mouse sperm heads by alkaline treatment. The protein profile of the PT extract was composed of several polypeptides, of which a 15-kDa subacrosomal protein predominated and was found to be immunocross-reactive with a previously cloned 15-kDa PT protein of the rat (PERF 15) that belongs to a family of lipid-binding proteins. A primer pair designed from rat PERF 15 cDNA was then used to screen a mouse testicular cDNA library by polymerase chain reaction (PCR). The deduced amino acid sequence obtained from the PCR product was almost identical to the testicular-specific rat PERF 15. Developmental Northern blots and in situ hybridization studies performed with riboprobes encoding the mouse PERF 15 cDNA revealed that mRNA levels were highest in round and early elongating mouse spermatids. Immunohistochemistry indicated that PERF 15 began to be expressed in the cytoplasm of mid-pachytene spermatocytes and appeared to reach maximum expression in the distal cytoplasm of late elongating mouse spermatids, long after transcriptional arrest. During the development of round and early elongated spermatids, the immunolabel became progressively concentrated over the anterior half of the spermatid nucleus, suggesting a subacrosomal deposition of PERF 15 during this phase of mouse spermiogenesis.