Histone H4 could be localized in rat spermatids using immunocytochemistry with a monoclonal histone-H4-antibody (IgM mouse) in combination with a goat-anti-mouse-antibody labeled with 6 nm gold particles. Label was found in spermatids from step 2 to step 9 in the nucleus predominantly near its envelope and in the electron-dense material of the chromatoid body. In later spermatids, where the chromatoid body had disappeared, gold particles were still seen in the nucleus but now evenly distributed over the condensed chromatin. In the controls without primary antibody both the nucleus and the chromatoid body were unlabeled. The results suggest that the chromatoid body might play a role in the nuclear protein transition during spermiogenesis.