An extract of normal chicken erythrocytes was found to be of unique importance as a constituent of the medium, both for intact parasitized cells and for lysates of such cells. Heated normal chicken serum possessed some favorable effect upon intact cells.

The major portion of the beneficial effect of the erythrocyte extract was found to be associated with the non-dialyzable portion, although the dialysate was shown to possess slight growth-promoting activity. Further attempts to characterize the extract in terms of its favorable effects upon parasite growth were unsuccessful.

A medium and conditions were developed which permitted the growth of a limited parasite population in lysates of parasitized red cells. The most important constituents of the medium were concentrated, freshly prepared erythrocyte extract and malate.

Comparative studies carried out with intact parasitized cells and with lysates showed that substances and conditions exerting quite marked effects upon the intact cell system were without effect or exerted a less marked effect upon parasite growth in lysates. The possible implications of these observations are discussed and the further exploitation of this comparative approach suggested as a means of furthering our knowledge of parasite-host cell relationships.