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The immobilization of paramecia when mixed with appropriate antisera has been utilized as a specific and semi� quantitative assay for the serotypic antigens (adsorption titration). While serotypic antigen in both soluble and particle�bound forms is detected by adsorption titration, only soluble antigen is detected in gel.diffusion analysis. 500 g, 3000 g, 12,000 g, and 105,000 g particulate fractions as well as 105,000 g and 150,000 g cell supernatant fractions were prepared by differential centrifugation. Analyses of these fractions indicate:

SUMMARYCell-free incorporation of amino acid into paramecium protein was accomplished by using ribosomes, soluble fraction, guanosine triphosphate and adenosine triphosphate. Less than 20% of the incorporated label was detected in the soluble fraction, indicating that little if any complete de novo synthesis occurred. Incorporation was markedly decreased by submicrogram concentrations of RNase and by decreasing [Mg2+] to below 3 ,umoles/ ml. A 'pH 5 fraction ' from mouse liver was able to replace the paramecium soluble fraction but attempts to obtain active 'pH 5 fractions' from paramecium failed. Evidence is presented for the presence of polyribosomes in paramecium; there was some indication that they were active in amino acid incorporation. Following incorporation, 80s ribosomes labelled with amino acid were recovered ; subjecting these to Mg2+-deficient buffer caused dissociation into 45s and 30s units. A considerable portion of the label remained with the heavier unit.

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