We report studies designed to establish optimal conditions for the assay of amniotic cell galactose 1-phosphate uridyl transferase (Gal-PUT) for early prenatal diagnosis of galactosaemia. Methods based on linkage of the reaction to cause of non-specific reactions occurring even in the absence of Gal-1-P. In the final method, sonicates of confluent cultures are incubated with (14-C) Gal-1-P is degraded by treatment with alkaline phosphatase. Gal-PUT specific activities of both control and galactosaemic amniotic cells are higher in non-confluent that confluent cultures.