Erythrocytes infected with schizonts of P. knowlesi and labelled in vitro with (3)H-isoleucine were shown to release radioactivity when placed in unlabelled culture medium containing uninfected erythrocytes. Release of radiolabel probably occurred during schizont rupture and invasion of new erythrocytes and was not observed to any great extent at earlier stages in the erythrocytic life cycle. Approximately 55% of the released radioactivity was soluble in 10% trichloroacetic acid, the remainder being acid insoluble and therefore presumably protein in nature. Particulate or aggregated material appeared to comprise 40-50% of the acid-insoluble radiolabelled fraction. Two major labelled polypeptide species were observed on sodium dodecyl sulfate polyacrylamide gel electrophoresis of samples of concentrated culture medium. These proteins had apparent molecular weights of 45 000 and 49 000, which correspond closely with the sizes of two main protein species previously identified in P. knowlesi-infected cells.