To understand the spermatid-specific regulation of the protamine-1 (Prm-1) gene, we examined the nuclear proteins that bind to regions of the Prm-1 promoter known to regulate transcription. We focused on the Prm-1 promoter region between bp -224 and -37 because this region directs spermatid-specific expression of a heterologous reporter gene in transgenic mice and because regulatory function has been demonstrated for several subregions of this fragment. Testis nuclear proteins that bind to this promoter region were identified by means of gel mobility shift assays, and the tissue distribution of these proteins was examined through nuclear extracts derived from several mouse tissues. Nuclear extracts prepared from prepubertal and mutant mouse testes were used to demonstrate the developmental appearance of these DNA-binding proteins during spermatogenesis. These studies indicate that a testis-specific protein that appears after Day 12 interacts with a sequence (between bp -37 and -77) shown to be essential for Prm-1 transcription in vivo. In addition, a number of proteins that are not restricted to the testis interact with other functionally important regions of the Prm-1 promoter.