Uncultured human leukocytes contain no detectable cystathionine synthase activity. A method is described in which the addition of phytohemagglutinin (PHA) to short-term lymphocyte cultures results in a significant induction of enzymatic activity. This PHA-stimulated activity has characteristics that resemble those previously described for cystathionine synthase of normal liver and cultured fibroblasts. Lymphocyte cystathionine synthase activity is completely dependent on the presence of homocysteine and is absent or severely deficient in extracts from individuals with the syndrome of homocystinuria. This system for induction of cystathionine synthase in lymphocytes thus provides a simple in vitro technique for (a) diagnosing homocystinuria, (b) studying the mechanism of enzyme regulation and differentiation, and (c) examining the nutritional and hormonal control of cystathionine synthase activity both in normal subjects and homocystinuric patients.