Utilizing the cytoplasmic tail of Transforming Growth Factor Receptor Type II (TGFbeta RII) as bait in a yeast two hybrid system, we have identified human cyclin B2 as a direct physical partner of TGFbeta RII. Analysis of deletion mutants of glutathione-S-transferase (GST)-cyclin B2 mapped its binding domain for TGFbeta RII to the C-terminal and revealed a negative regulatory region immediately upstream of the cyclin box. Using recombinant proteins, Cdc2 was demonstrated to indirectly interact with TGFbeta RII via cyclin B2. This interaction was reproduced in THP-1 monocytic cells, where TGFbeta treatment markedly enhanced the ability of cyclin B2 and, correspondingly, Cdc2 from TGFbeta-treated THP-1 cells, to bind the GST-TGFbeta RII fusion protein. More importantly, TGFbeta RII co-precipitated with cyclin B2 in TGFbeta-treated THP-1 cells. TGFbeta treatment also caused threonine phosphorylation of Cdc2 in the TGFbeta RII-cyclin B2-Cdc2 complex in THP1 cells, in parallel with down regulation of Cdc2 function as measured by histone H1 kinase activity. Cyclin B1 had the same capacity to bind TGFbeta RII and mediate indirect Cdc2 binding. These results suggest an alternative mechanism that cell cycle arrest in the G1/S phase caused by TGFbeta may, in part, be due to inactivation of cyclin B/Cdc2 kinase, which is needed for entry into the G2/M phase.