Proteins homologous to the Xenopus oocyte mRNA binding proteins mRNP3+4 and designated p48/52 have been identified in male mouse germ cells (1993: Dev Biol 158:90-100). Western and North-western blots of extracts from testes and isolated germ cells indicate that p48/52 are present during meiosis but reach their highest levels postmeiotically at a time when many mRNAs are stored. Here we analyze the cellular and subcellular distribution of p48/52 in rat and mouse testes by LM and EM immunocytochemistry using an anti-mRNP3+4 antibody. Immunolabeling was found to be predominantly cytoplasmic and specific to germ cells at certain periods during their development. p48/52 were first detected in early pachytene spermatocytes at stage V of the seminiferous cycle and progressively increased during the remainder of meiotic prophase to a post-meiotic peak in steps 1-8 round spermatids; thereafter, labeling gradually declined as elongated spermatids underwent nuclear condensation and elongation. A proportionally higher concentration of cytoplasmic immunolabeling was found within the lacunae of the anastomotic granulofilamentous network of the chromatoid body. The pattern of synthesis of these mRNA binding proteins together with their association with the chromatoid body suggests a role as germ cell-specific mRNA stabilizing and/or storage proteins.