1. Properties of porphobilinogenase (PBGase), the enzyme complex converting porphobilinogen (PBG) into uroporphyrinogens, were studied in a wild strain, D273-10B and a mutant, B231, of Saccharomyces cerevisiae. 2. A well-defined maximum of enzyme activity was observed for the mutant strain after 20 hr of growth; whilst the activity in the wild strain did not vary significantly during growth. 3. Neither PBG consumption nor uroporphyrinogen formation were modified by the presence of air either in the wild or in the mutant strain. 4. In both the wild and mutant strains uroporphyrinogen formation increased linearly with both protein concentration and incubation time. 5. The addition of a mixture of sodium and magnesium salts to the assay system inhibited the enzyme activity of both strains by 50% without modifying the isomer composition. 6. The same optimum pH (7.4) and mol. wt (50,000 +/- 5000) was found for the enzyme from both strains. 7. The enzyme from both the wild and mutant strains shows Michaelis-Menten kinetics when isolated from cells at either the exponential or the stationary phases of growth. Accumulation of porphyrins and delta-aminolevulinic acid occurring during the exponential phase in the mutant strain, did not modify the kinetics.