SUMMARY Cell-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 submicro- gram 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 para- mecium 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 disso- ciation into 45s and 30s units. A considerable portion of the label remained with the heavier unit.