N-Bromosuccinimide cleavage of in vivo 32P-labelled lysine-rich histone isolated from rapidly dividing cells has been studied. N-Bromosuccinimide cleaves F1-histone into two fragments, a small N-terminal piece and a larger C-terminal portion. The phosphate-induced microheterogeneity and associated radioactivity which has been linked to cell replication, is found in the carboxyterminal fragment, No phosphorous is found associated with the amino-terminal fragment when histone phosphorylation is associated with cell division. The specific tryptic phosphopeptides obtained from in vivo labelled F1 are clearly different from those obtained from in vitro incubations of free F1-histones and cytoplasmic protein kinase.