The yeast Saccharomyces cerevisiae contains four proteins having amino acid compositions typical of the high mobility group (HMG) proteins. Three of these are eluted from chromatin by 0.35 M NaCl; one is not, but it is eluted by 0.25 N HCl. It follows that HMGs cannot, in general, be defined by extractability criteria. Gel mobilities and amino acid compositions indicate that yeast and animal HMGs have diverged markedly.