Assay of galactose-I-phosphate uridyl transferase in erythrocytes has been applied to a study of 14 families in which a known case of galactosemia has occurred. Including normal controls, 278 individuals were involved. The method employed was sensitive enough to differentiate between homozygous, heterozygous and normal individuals. The results substantiate the conclusion that galactosemia is a genetic disease transmitted by simple autosomal Mendelian inheritance.