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Labile altered forms of the his4 protein from Saccharomyces cerevisiae were studied in extracts of mutant strains or in extracts subjected to proteolysis. Fragments of the his4 protein were detected by immunoautoradiography in high salt extracts of his4 deletion strains having histidinol dehydrogenase activity but lacking two of the other enzyme activities associated with the trifunctional his4 protein. A form of the protein altered by proteolysis was also detected in low salt crude extracts by immunodiffusion and shown to possess histidinol dehydrogenase function by activity staining of the precipitin lines. These immunological data indicated that histidinol dehydrogenase can function independently of the other two activities in extracts of deletion strains or in extracts where the wild type his4 protein has undergone proteolytic cleavage. The data also indicated that high concentration in (NH4)2SO4 had a substantial stabilizing effect on the his4 protein and on fragments of the his4 protein from mutant strains.