The heterogeneity of calf histones is limited. The histones are divided into five major electrophoretic groups, several of which are further subdivided to make a total of twelve species of histone molecule. The heterogeneity described is probably not due to impurity (though this is hard to assess), to failure of extraction, to degradation during extraction, or to multiple polymerization through disulfide bonds. The relative amounts of each of the twelve components is reported. No significant differences in relative quantitation are observed for bovines of varying age and sex. A tissuespecific lysine-rich histone is reported; other major groups of histone show tissue specificity only in terms of variation in the amount of each histone fraction present. © 1969, American Chemical Society. All rights reserved.