The molecular weight forms of kynurenine formamidase were studied both gentically and biochemically. Formamidase I (native molecular weight 60,000) was purified using (NH4)2SO4 and pH fractionation, DEAE-cellulose chromatography at two different pH's, hydroxylapatite chromatography, and Sephadex G-100 gel filtration. Its subunit molecular weight, as determined by SDS gel electrophoresis, is 34,000, indicating that formamidase I is a dimer. Its Km is 1.87 X 10(-3) M. Its isoelectric point is pH 5.3. Its amino acid composition is reported. Formamidase II (native molecular weight 31,000) was partially purified using techniques similar to those above. Its Km is 2.31 X 10(-3) M. The response of formamidase activity to change in gene dosage was measured in segmental aneuploids generated in the second, third, and X chromosomes. Two separate chromosomal regions were identified which when present in extra dosage result in an elevation of the level of formamidase activity close to that predicted for the addition of a structural gene in a two-gene system. These tentative map positions were substantiated by demonstration that addition of one of the regions, 25A-27E, causes a 50% elevation in the relative amount of formamidase II. Addition of the other region, 91B-93F, causes a similar elevation in the relative amount of formamidase I. A model of the evolutionary origin of the two forms is presented, and the significance of these results to this model is discussed.