In humans, intermediate basic proteins HPI1 and HPI2 are considered as common precursors of the P2 protamine family, according to data provided by structural studies of these proteins. The occurrence and fate of proteins HPI1 and HPI2 were investigated in nuclei of human spermatids and spermatozoa by means of immunoelectron microscopy. A specific polyclonal antibody against a synthetic peptide overlapping the N-terminus of HPI1 and HPI2 was prepared and used to detect these proteins on sections of testis and ejaculated sperm. A quantitative analysis of labelling density was performed on micrographs using an interactive image analysis system. The first signs of labelling of intermediate basic proteins appeared in spermatid nuclei at steps 4-5 of spermiogenesis, i.e. during the chromatin condensation process. The nuclear labelling density strongly increased in elongating spermatids (steps 5 and 6) and then sharply decreased from step 6 to step 8 of spermiogenesis. However, weak labelling persisted in the nuclei of mature spermatids and ejaculated spermatozoa. The present results show that the intermediate basic proteins HPI1 and HPI2 are synthesized in large amounts in human spermatids during elongation phase and disappear almost totally in mature spermatids when deposition of protamines is completed in condensed nuclei.