BACKGROUND:Histone to protamine exchange, in man, is only 80% complete and spermatozoal histones are highly acetylated suggesting that their associated genes may be involved in early gene expression in the embryo. METHODS:Using in situ hybridization and immunocytochemistry, we analyzed expression of protamine-1 and protamine-2, and histone H4 specifically acetylated at lysine 5 (H4K5ac), 8 (H4K8ac), 12 (H4K12ac) and 16 (H4K16ac) in human (n = 22) and marmoset (n = 6) testes and ejaculates. RESULTS:Protamine-1 and protamine-2 mRNA was present in round and elongating spermatids. All antibodies against acetylated histones revealed positive signals in these cells. Human spermatogonia showed positive signals for H4K8ac and H4K16ac, whereas marmoset spermatogonia were positive for H4K8ac and H4K12ac. In man, H4K16ac already displayed a positive immunoreaction with pachytene spermatocytes, starting at stage III of the seminiferous epithelial cycle. All antibodies showed positive immunostaining in ejaculated spermatozoa of both species. CONCLUSIONS:The common marmoset represents a suitable animal model for studies on nuclear protein expression during human spermatogenesis. The two species exhibit a similar organization of seminiferous epithelium and an identical expression pattern of protamine-1 and protamine-2 mRNA in round and elongating spermatids. The presence of specifically acetylated histones H4 in testicular spermatids and ejaculated spermatozoa demonstrates an incomplete histone to protamine exchange in both species. As acetylated histones are known to be associated with genes involved in gene expression, the common marmoset may, in future, be used as a model for investigations on early embryo development which, in man, are not possible for ethical reasons.