Serological and biochemical studies were performed on M components isolated from 3 patients with transitional cell carcinoma of the urinary bladder (TCC). All were IgG1(K) proteins. 2 of the 3 belonged to the VKI subgroup, and these 2 also possessed cross-reactive idiotypic determinants. These findings are consistent with the ideas that M components in patients with a given kind of cancer may have restricted heterogeneity, and that the synthesis of an M component and the manifestation of a solid tumor may be related events in some patients who exhibit both abnormalities.