Abstract 1. 1. Conditions for the in vivo incorporation of dl-leucine-1-C 14 into the proteins of Trypanosoma cruzi are reported in this paper. Best results were obtained with KC1 0·154 M as suspension medium as compared to 0·25 M sucrose. 2. 2. Optimal pH in the absence of glucose was 7·6, while in its presence was 8·0. 3. 3. The addition of glucose increases the incorporation of the isotope into total proteins, while various metabolites of the Krebs cycle have an opposite effect. 4. 4. Cell fractionation studies indicate a rapid labeling of microsomal protein. Chromatography and radioautography show that isoleucine and valine are also labeled in the protein hydrolysate. 5. 5. Our results suggest that an active mechanism, similar to those reported for higher organisms, might be involved in protein biosynthesis in T. cruzi.