Prenatal exclusion of homocystinuria (cystathionine beta-synthase deficiency) by assay of phytohaemagglutinin-stimulated fetal lymphocytes.

Homocystinuria due to cystathionine beta-synthase deficiency was excluded in a fetus at 23 weeks' gestation by demonstrating activity of the enzyme in fetal lymphocytes after stimulation by phytohaemagglutinin. Fetal blood sampling was carried out because two determinations of enzyme activity on cultured amniotic cells gave low, not fully diagnostic values.