Peripheral lymphocytes were cultured in the presence and absence of phytohemagglutinin (PHA). Porphyrin synthesis was visually estimated by fluorescence and quantitated by fluorometry. No visual fluorescence was noted in cells cultured with or without PHA. Addition of δ-aminolevulinic acid (ALA) to cultures of either transformed or nontransformed lymphocytes resulted in fluorescence, indicating porphyrin synthesis. Certain sex steroid metabolites stimulate porphyrin synthesis by induction of the rate-limiting enzyme ALA synthetase. Erythropoietin also increases the rate of heme synthesis. Phytohemagglutinin-transformed and nontransformed cells were cultured with the addition of either 11-keto pregnanolone or erythropoietin, and neither agent induced porphyrin synthesis. Only when ALA was added to the culture medium was the measured fluorescence statistically greater than in control cultures. These studies indicate that the enzymes of porphyrin synthesis distal to ALA synthetase, the rate-limiting enzyme, are present in all cultured lymphocytes. The rate-limiting enzyme is not induced by PHA transformation, even in the presence of a 5-β-H steroid metabolite or erythropoietin.