We recently reported that cultures of terminally differentiating myotube cells synthesize histones in reduced but significant amounts in comparison with proliferating myoblasts (Wunsch et al., 1987, Dev. Biol., 119: 85-93). In this study, the stability of myotube histone has been determined, comparing the degradation of de nov-synthesized histones in nascent (day 3 ) and maturing (day 4) myotubes with histones in the same cells that had been previously made during myoblast proliferation (day 1). Histones synthesized in proliferating myoblasts and myotubes were pulse-labeled with 3H-lysine and chased up to seven days, followed by determinations of radioactivity remaining in histone bands using fluorography of one-and two-dimensional polyacrylamide gels. Considered in aggregate, core histones synthesized de novo in nascent (day 3 ) myotubes were degraded most rapidly, followed by myotube histones that had been previously made during the proliferative phase (day 1 ) of myogenesis. De novc+synthesized histones in maturing (day 4) myotubes were relatively stable. Individual histone classes were degraded in the following order of increasing half-life, regardless of the differentiatiie stage at which they H3(.2, day 1; .3, days 3 and 41, H4.