It was shown by gel electrophoresis in sodium dodecylsulphate solution that 3-methylcrotonyl-CoA carboxylase from Achromobacter IVS is composed of two different subunits with molecular weights of about 78000 and 96000, respectively. The biotin is bound to the heavier subunit. It was previously found that 3-methylcrotonyl-CoA carboxylase contains four biotin molecules per complex. A complex composed of four of each subunit would thus have a molecular weight of about 700000. This is compatible with the molecular weight of 760000 determined earlier by analytical ultracentrifugation. Both subunits were isolated preparatively. As the subunits, unlike the complex, are very sensitive to oxygen, special precautions had to be taken during isolation. The biotin-containing subunit was isolated by chromatography on DEAE-cellulose in 5 M urea. It no longer catalyzed the overall reaction, yet could still carboxylate free biotin. The biotin-free subunit was separated after dissociation of the enzyme by three-days' dialysis at pH 9.8 under nitrogen. On chromatography over a Sepharose-bound avidin column, the biotin-subunit was fixed and the biotin-free subunit was eluted unretarded. The latter subunit showed no enzymic activity. After the addition of the biotin-containing subunit, overall activity was regenerated. The speed of reassociation is very much enhanced by 3-methylcrotonyl-CoA. It was shown by reassociation experiments under different conditions that probably an initial complex, AxBy is formed, possessing a binding site for 3-methylcrotonyl-CoA. Upon the binding of this substrate the conformation may be changed to a form favourable for reconstitution. Finally, the structures of biotin enzymes from different sources are compared. In the course of evolution there is a tendency toward integration of the different constituent proteins into only one polypeptide chain.