A histone-type protein kinase (EC 2.7.1.37) has been partially purified (320-fold) from the crude extracts of soybean hypocotyls by means of a combination of gel filtration and anion exchange procedures. The purified enzyme fraction is devoid of the activities of phosphoprotein phosphatase (EC 3.1.3.16), histone protease, and casein (or phosvitin)-type kinase. The soybean histone kinase uses ATP to phosphorylate specifically lysine-rich histone H1 from either pea seedlings or calf thymus.The histone kinase requires free sulfhydryl group(s) for activity, but not stability. The pH optimum is around 9 to 10. The apparent K(m) values for histone H1 of pea seedlings and calf thymus are 0.4 and 0.9 micromolar, respectively. The K(m) values for ATP are 40 nanomolar with the optimal concentration of Mn(2+) (50 nanomolar) and 0.4 micromolar with that of Mg(2+) (5 millimolar). The estimated molecular weight of the kinase is 52,000 by gel filtration or 48,600 by sedimentation constant (3.2 S). cAMP does not alter the sedimentation velocity of the kinase. The enzyme activity is unaffected by cyclic nucleoside monophosphates and plant growth substances. Like arginine-rich histones, a variety of divalent cations and polycations (polyamines) are inhibitory.This cAMP-independent soybean histone kinase is not associated with the isolated ribosomes but shows highest specific activity in the nuclearchromatin fraction, suggesting that it may function in the regulation of histone H1 phosphorylation in the soybean hypocotyl.