Nucleosomal semihistone A24 (uH2A) is composed of histone H2A and ubiquitin peptides. The kinetics of incorporation of nascent H2A and ubiquitin into A24 of transformed chicken lymphocytes (MSB cells) have been examined by peptide mapping, COOH-terminus analysis, density labeling and isopycnic centrifugation of chromosomal proteins. We find that newly synthesized H2A is rapidly conjugated to ubiquitin. SDS-PAGE analysis of fractionated density gradients suggests, however, that newly synthesized ubiquitin becomes bound nonselectively to both new and preexisting H2A molecules.