Electrophoretic analysis of acid-soluble chromosomal proteins isolated from rooster testis cell nuclei at different stages of spermatogenesis, revealed that the nuclear content of a protein identified by its solubility, electrophoretic mobility and amino acid analysis as the protein conjugate histone H2A-ubiquitin (uH2A, A24) changed markedly from meiotic cells to late spermatids. The protein was not detectable in tetraploid primary spermatocytes; it was present in 1.7% of the total amount of nucleosomal core histones in early spermatids and reached its maximum level (3.5% and 11%) at the end of spermiogenesis, when histones are replaced by the protamine galline.