A complex of orotate phosphoribosyltransferase and orotidylate decarboxylase has been shown to exist in three molecular weight forms (Brown, G. K., Fox, R. M., and O'Sullivan, W. J. (1975), J. Biol. Chem. 250, 7352). The smallest of these, of molecular weight 62 000, was subjected to further study. On the basis of the inactivation of the enzyme activities, carried out in the presence of low concentrations of guanidine hydrochloride, and of changes in molecular weight of preparations during aging, it was inferred that the enzyme complex contained more than one type of subunit. This was confirmed by chromatography on Sephadex G-75 after preincubation in guanidine hydrochloride or with guanidine hydrochloride in the elution buffer. It was concluded that the enzyme complex consisted of two types of subunits, two decarboxylase units of molecular weight approximately 20 000 and two further subunits of approximately 13 000. The subunits could be separated and reassociated with partial recovery of both activities. A 40 000 molecular weight form had full decarboxylase activity but no phosphoribosyltransferase activity. Restoration of the 62 000 molecular weight form resulted in restoration of both enzymatic activities. An intermediate species of molecular weight 50 000 representing a combination of the decarboxylase dimer with one of the 13 000 subunits was also demonstrated. This form required the presence of dithiothreitol in order to manifest phosphoribosyltransferase activity. A model of the system has been proposed that accounts for both the different molecular weight forms and also for the deficiency of both activities in the rare inborn error of metabolism, hereditary orotic aciduria.