Protamine-like sequences have been identified in poly A(+) mRNAs from mammalian testes by the use of a common, complementary oligonucleotide (GCAGCANCK PTANCKNGCCAT; predicted from the common N-terminal amino acid sequence, MARYRCC, seen in several mammalian P1 protamines [D. J. McKay, B. S. Renaux and G. H. Dixon, Bioscience Reports5:383–391 (1985)]). This oligonucleotide was utilized to prepare species-specific, primer-extended transcripts for use as Northern blotting probes. Analysis of the mRNA primer-extended transcripts revealed a discrete and similar set of products common to both bull and rat testis mRNAs which were distinct from those obtained from human testis mRNA. Northern analysis of total poly A(+) mRNAs from the corresponding mammalian testis was consistent with these results and suggests that bull and rat protamine mRNAs are more closely related to each other than to human protamine mRNA.