Four subfractions of histone F1 from rat thymus were obtained by preparative polyacrylamide-gel electrophoresis. These subfractions are closely related in primary structure but show marked differences in radioactivity when derived from rat thymus nuclei labelled in vitro with [ 32 P]phosphate.
1. Two minor protein fractions (B and C) were separated from histone F1 by chromatography on DEAE-cellulose. Fraction B was acidic. 2. Uptake of 32 P in vivo into histone F1 but not into fractions B and C was stimulated by partial hepatectomy. 3. It is suggested that partial hepatectomy causes an increase in the number of histone F1 molecules phosphorylated.