We have previously detected and purified a Friend erythroleukemic mouse cell nonhistone chromatin protein having extraction and acid-solubility properties like the low molecular weight "high mobility group" (HMG) nuclear proteins. We show here that the electrophoretic properties and the amino acid composition of this mouse cell "HMG-like" protein is comparable to those of the HMG-I proteins isolated from human HeLa S3 cells, African green monkey cells, Ehrlich ascites mouse cells, and rat fibroblast cells. Therefore, we have also designated the Friend erythroleukemic mouse cell protein as HMG-I. In common with the other HMG proteins the Friend cell HMG-I protein can undergo a variety of post-translational biochemical modifications including acetylation, ADP-ribosylation, glycosylation, and phosphorylation. Surprisingly, in the course of these studies we found that in vivo radiolabeling experiments revealed that only two minor HMG-14 subspecies (and/or possibly a minor HMG-I subspecies) are phosphorylated whereas HMG-1, -2, -17, and the major HMG-14 are not heavily phosphorylated.