Protamines P1 and P2 form a family of small basic peptides that represent the major sperm proteins in placental mammals. In human and mouse protamine P2 is one of the most abundant sperm proteins. The protamine P2 gene codes for a P2 precursor, pro-P2 which is later processed by proteolytic cleavages in its N-terminal region to form the mature P2 protamines. We have used polymerase chain amplification to directly sequence the pro-P2 genes of the five major primate families: red howler (Alouatta seniculus) is a New World monkey (Cebidae); the two macaque species, Macaca mulatta and M. nemistrina are Old World monkeys (Cercopithecidae), the gibbon, Hylobates lar, represents one branch of the apes (Hylobatidae); the orangutan, Pongo pygmaeus, gorilla, Gorilla gorilla and two species of chimpanzee Pan paniscus and Pan troglodytes represent a second ape family (Pongidae). These pro-P2 genes are compared with that of human [Domenjoud, L., Nussbaum, G., Adham, I. M., Greeske, G. & Engel, W. (1990) Genomics 8, 127-133]. The overall size and organization of the genes are conserved within the group. The mean length of pro-P2 is 101 residues, with an increase to 102 in M. nemistrina and a decrease to 99 residues in red howler (A. seniculus). In gorilla and red howler one of two 79-bp tandem repeats that occurs 3' of the gene is deleted. Of the 101 deduced amino acids examined, an amino acid change occurs in one or more primates at 45 positions. Considering only the most recently diverged group, the human/gorilla/chimpanzee clade, this represents a very high mutation rate of 0.99 changes/100 sites in 10(6) years. This rapid mutation rate is characteristic of both members of the protamine gene family, P1 and P2. Consideration of the variable nature of the sequences at the multiple sites of proteolysis during the processing of the pro-P2 indicates either that there are several processing enzymes of differing specificities, or more likely that the folded structure of the pro-P2 limits accessibility of a non-specific protease to certain exposed sites.