Juraj Osterman and Paul J. Fritz* abstract: Five electrophoretically distinct zones of pyruvate kinase activity were observed in extracts of rat jejunal and ileal mucosa. The major form is pyruvate kinase 5 corre-sponding to the predominant isozyme in rat kidney and the minor isozyme in rat liver. Four of the five bands are neu-tralized by rabbit anti-rat pyruvate kinase 3(type M). Pyruvate kinase 1(type L) was not neutralized. Onlyfour peaks with pyruvate kinase activity could be resolved by DEAE-Sephadex chromatography and the first threepeaks had the electrophoretic mobility of pyruvate kinase 5. The fourth peak contained two electrophoretically distinct bands, one with the mobility of pyruvate kinase 1 and the otherwith a mobility between pyruvate kinase 2 and 3. Rat jejunal