Phosphorylation of the retinoblastoma protein (RB) was observed during apoptosis of B50 neuroblastoma cells following induction by dibutyryl cAMP, after differentiation into neurons, or by cycloheximide during proliferation. A weak but distinct increase in a RB and histone H1 kinase activity was detected at the time of RB phosphorylation. However, the RB kinase appeared to correspond to neither p34cdc2 kinase, CDK2 nor CDK5 because it was not inhibited by butyrolactone I, an inhibitor for them. Expression of CDK4 and 6 along with several cyclins also did not coincide with the appearance of phosphorylated RB in the apoptotic process.