Histone H1 kinase activity in polyploid MKs. Immunoprecipitation of cyclin B1 in the two fractions of MKs synchronized by nocodazole and sorted on their ploidy (2N-4N and >4N) or in asynchronized MKs was performed with an MoAb raised against cyclin B1 (GNS-1; Pharmingen). The histone H1 was phosphorylated by the immunoprecipitate in the presence of (γ-³²P)ATP and the phosphorylation was determined by electrophoresis on a 12% polyacrylamide gel followed by autoradiography. The results were compared with a negative control [(γ-³²P)ATP, histone H1 with control IgG1 immunoprecipitate]. Lane 1, (γ-³²P)ATP histone H1 with control IgG1 immunoprecipitate as a negative control; lane 2, 2N-4N MKs; lane 3, >4N MKs; lane 4, unfractionated MKs (5 × 10⁶ cells); lane 5, unfractionated MKs (1 × 10⁶ cells); lane 6, unfractionated MKs (5 × 10⁵ cells).