Transforming growth factor-beta (TGF-beta) transduces signals through its type I and type II receptors. Both receptor types have previously been shown to interact in a heteromeric complex in the presence of TGF-beta. We have now characterized these interactions between both receptor types using a combination of yeast two-hybrid interaction assays and coimmunoprecipitation analyses. Our results indicate a direct association between the cytoplasmic domains of the two receptor types. Mutation analysis of these cytoplasmic domains reveals that this direct interaction requires kinase activity and, thus, depends on phosphorylation, probably via a transphosphorylation mechanism. Furthermore, the two receptor types already have an inherent affinity for each other in the absence of TGF-beta, and the heteromeric complex can be detected in coimmunoprecipitations under these conditions. Taken together, our results reveal a novel mechanism of receptor complex formation, whereby two different cytoplasmic domains directly associate with each other. This interaction may play a major role in activation of serine/threonine kinase receptors.