ExtractCystathionine synthase activities are reported for extracts of fibroblasts grown from 39 control subjects, 47 homocystinuric individuals, and 10 parents of cystathionine synthase-deficient patients. Among the group with homocystinuria, fibroblast extracts from 38 had specific activities of cystathionine synthase below the control range. A number of considerations indicate that these 38 patients excrete homocystine because of cystathionine synthase deficiency. Fibroblasts from nine patients with homocystinuria had specific activities of cystathionine synthase within the control range. This group of nine was shown to be comprised of two individuals with cystathionine synthase deficiency, three with deficient activity of methylenetetrahydrofolate reductase, three with deficient activity of iV 5 -methyltetrahydrofolate-homocysteine methyltransferase, and one in whom homocystine excretion found by others in earlier studies could not be confirmed during the present investigation. The specific activities in fibroblasts of parents of cystathionine synthase deficient patients in most cases fall near the low end of the control range.An assay of increased sensitivity was used to measure the cystathionine synthase activities in extracts of fibroblasts from cystathionine synthase-deficient patients at several concentrations of added pyridoxal phosphate. Of 25 cystathionine synthase-deficient patients judged to be clinically responsive to pyridoxine treatment, 24 had detectable cystathionine synthase activities in fibroblast extracts when the assays were performed without added pyridoxal phosphate. These activities ranged from approximately 0.1 % to 10% of the mean control value, and generally were stimulated no more by the addition of pyridoxal phosphate than were extracts from normal cells. Of 10 cystathionine synthase-deficient patients judged not to be responsive to pyridoxine, 9 had no cystathionine synthase activity in fibroblast extracts detected by the method employed. The cystathionine synthase in the cell extracts of the single nonresponsive patient with significant activity was stimulated much more by in vitro addition of pyridoxal phosphate than was control cystathionine synthase.Genetic heterogeneity in cystathionine synthase-deficient patients and some of the genetic implications of the demonstrated a 2 /32 subunit structure of mammalian cystathionine synthase are discussed. Speculation