During phytohemagglutinin and concanavalin A-induced transformation of human lymphocytes, phosphoglycerate kinase (PGK) exhibits new electrophoretic forms (pI = 8.5–8.9). Electrophoresis and electrofocusing showed that the new forms are not due to expression of the autosomally linked isozyme found in sperm (PGK-B; pI = 9.7). The multiple electrophoretic forms are the result of protease modification of the sex-linked PGK-A isozyme. © 1980, All rights reserved.