1. The activity of the soluble phosphokinase for histone F1 increases in regenerating rat liver during the first period of DNA synthesis after partial hepatectomy. The increase probably represents new enzyme synthesis. 2. A dose of 500rd of γ-irradiation given early in G1 decreases the amount of histone F1 phosphokinase found 22h after partial hepatectomy by 60–70%. 3. The enzyme preparations also contained a histone F1 phosphatase; the presence together of the kinase and phosphatase caused a disproportion between net 31P uptake and 32P incorporation into histone F1. 4. All four subclasses of histone F1 could accept phosphate from ATP. 5. Crude enzyme preparations transferred more 31P into histone F1 with an initially low phosphate content than into one with a high phosphate content; conversely, more 32P was transferred into the latter.

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