Gadd45a (Gadd45), Gadd45b (MyD118), and Gadd45g (CR6) constitute a family of evolutionarily conserved, small, acidic, nuclear proteins, which have been implicated in terminal differentiation, growth suppression, and apoptosis. How Gadd45 proteins function in negative growth control is not fully understood. Recent evidence has implicated Gadd45a in inhibition of cdc2/cyclinB1 kinase and in G2/M cell cycle arrest. Yet, whether Gadd45b and/or Gadd45g function as inhibitors of cdc2/cyclinB1 kinase and/or play a role in G2/M cell cycle arrest has not been fully established. In this work, we show that Gadd45b and Gadd45g specifically interact with the Cdk1/CyclinB1 complex, but not with other Cdk/Cyclin complexes, in vitro and in vivo. Data also has been obtained that Gadd45b and Gadd45g, as well as GADD45a, interact with both Cdk1 and cyclinB1, resulting in inhibition of the kinase activity of the Cdk1/cyclinB1 complex. Inhibition of Cdk1/cyclinB1 kinase activity by Gadd45b and Gadd45a was found to involve disruption of the complex, whereas Gadd45g did not disrupt the complex. Moreover, using RKO lung carcinoma cell lines, which express antisense Gadd45 RNA, data has been obtained, which indicates that all three Gadd45 proteins are likely to cooperate in activation of S and G2/M checkpoints following exposure of cells to UV irradiation. © 2002 Wiley‐Liss, Inc.