1. Immunosorbents were prepared by coupling activated aminocellulose with the gamma-globulin concentrates of antisera prepared against ovalbumin and human serum albumin. 2. The immunosorbents were low in solubility, but high in capacity for homologous antigens. 3. The high specificity of these immunosorbents was demonstrated by their use in fractionating various mixtures of fluorescent ovalbumin, (131)I-labelled human serum albumin, lysozyme and ribonuclease.

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I956 methionine in its protein is derived from that in the red-cell globin. 2. The conversion by P. knowle8i of [35S]methionine into [35S]cystine has been shown to occur in vitro, and this provides a source of cystine sulphur for the parasite in addition to that already present in the haemoglobin and glutathione of the erythrocyte. Some estimates of the extent to which these sources contribute to the cystine requirement of the parasite have been made. 3. Under identical in vitro conditions it was found that the uptake of DL-[35S]methionine by parasitized erythrocytes was markedly greater than that of DL-[35S]cysteine. 4. The amounts of methionine and cystine present in the haemoglobin of a monkey erythrocyte are respectively 6-0 and 2-4 times that found in the protein of a mature parasite. The implications of this observation to the nutrition of the parasite have been discussed.

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