A method has been devised for the measurement of uroporphyrinogen I synthetase ih red cells. By using trichloroacetic acid as a protein precipitant, heme is removed from the final solution, allowing accurate measurement of porphyrins. The method is highly reproducible and adaptable to varying incubation volumes and enzyme preparations. It is of great value as an enzyme diagnostic method for acute intermittent porphyria and appears capable of detecting patients with the latent disease who have normal urinary δ-aminolevulinic acid and porphobilinogen excretion. It also appears to distinguish other types of porphyria from acute intermittent porphyria. The mean value of the enzyme in red cells of patients with acute intermittent porphyria was approximately 50% that of normals, indicating that the mutation causes complete lack of catalytic activity in the mutant enzyme.