A method is described for studying the incorporation of radioactive amino acids by leukemic leukocytes in vitro. The effects of time of incubation, concentration of amino acid, temperature, pH, oxygen tension, electrolytes, and glucose concentration on the uptake of S35 L-cystine and S35 L-methionine were studied. The in vitro incorporation of these amino acids appears to be an active process and not solely dependent upon simple diffusion. Reproducible results were obtained under constant conditions. The methods are suitable for studying possible therapeutic agents as well as biochemical abnormalities of leukemic leukocytes.