We have investigated the association of non-tubulin microtubule proteins with tubulin to form the ring-shaped oligomeric structures found in microtubule preparations. We have found that the two oligomeric species present in our preparations of microtubule protein (s020,w = 18 and 30 S) each require non-tubulin factors for their formation. Two types of non-tubulin protein, the high molecular weight proteins (HMW) and the tau proteins were found to be active in ring formation. The HMW proteins promoted the formation of the 30 S oligomer, while the tau proteins promoted the formation of an oligomer of s020,w = 20 S. Analysis of the 30 S oligomer by gel filtration chromatography showed that the ratio of HMW proteins to tubulin was about twice that in the microtubule. The HMW proteins could be destroyed by exposure to trypsin, resulting in a marked increase in the sedimentation coefficient of the 30 S oligomer to 39 S. The 20, 30, and 39 S species were identified as rings by electron microscopy. The identity of the 18 S structure as a ring was called into question. Our data indicate that the 20 S species is a single ring and that the 30 S oligomer is a two-layered ring bearing HMW projections which contribute substantial hydrodynamic drag to the particle. We compare the organization of tubulin subunits and HMW molecules in the 30 S ring with the organization of these components in the microtubule and suggest that the organization in the ring is conserved in the microtubule.