Enzymatic studies of liver extracts from patients with homocystinuria due to cystathionine synthase (synthase) deficiency gave evidence of genetic heterogeneity. Two B₆-unresponsive patients had no detectable synthase activity. Four B₆-responsive patients all had easily measurable activity prior to treatment; three of them had approximately fourfold increases in activity following treatment, the fourth showed no clear-cut increase in activity. A seventh patient for heterozygote), who presented an incomplete phenotype, had 15 per cent of mean control synthase activity, which increased twofold following a methionine load and fourfold after pyridoxine treatment. Four obligate heterozygotes had synthase activities of 21, 31, 29, and 46 per cent, respectively, of mean control, although the last was assayed after a methionine load. Hepatocytes from all the patients showed the abnormalities by electron microscopy previously described in unresponsive patients. The morphologic abnormalities were not significantly changed after treatment even in the B₆-responsive patients. Hepatocytes from the four obligate heterozygotes contained the same morphologic changes.