Larval Taenia crassiceps has an active, cell-free protein synthesis system. The system, which consists of polysomes and a pH 5 fraction, is dependent on ATP, GTP, Mg2+, K+, and an ATP-generating system and is sensitive to RNase and puromycin under optimal conditions. No activity is observed in the absence of polysomes. Soluble factors appear to be tightly bound to polysomes since 50% of the activity is obtained when the pH 5 fraction is omitted. Electron microscopy of total polysomes and polysomal fractions obtained by sucrose density gradient centrifugation indicates the presence of ribosomes and polyribosomal aggregates of 2 to 6 units and higher, the latter predominating. Polysomes may be labeled in vivo either by injecting the host with a labeled amino acid precursor or by incubating the isolated parasites with the amino acid. Polysomes and the pH 5 fraction from T. crassiceps are exchangeable with polysomes and the pH 5 fraction from mouse liver; however, the activity of T. crassiceps polysomes supplemented with the mouse liver pH 5 fraction is somewhat lower than that of the homologous system. Rat liver polysomes show very little activity in the presence of T. crassiceps pH 5 fraction, but T. crassiceps polysomes show significant activity with rat liver pH 5 fraction. © 1977.