Maturation promoting factor, (MPF), is a crucial regulatory component of the eukaryotic cell cycle. Though it is ubiquitous, MPF has been difficult to purify to homogeneity, and little is known about its physical properties or composition. In an attempt to further characterize and purify this protein, we have isolated five monoclonal antibodies that immunoadsorb MPF activity, and inhibit the activity in solution. However, all the antibodies recognize many proteins in partially purified MPF. We have shown that antibody binding is dependent on previous exposure of the preparation to ATP gamma S. This suggests that the antibodies specifically recognize thiophosphoproteins, although not all thiophosphorylated proteins in MPF are immunoprecipitated. Using one antibody, MPF was partially purified by immunoadsorption chromatography. These experiments provide the first evidence that MPF from Xenopus is a phosphoprotein that becomes thiophosphorylated upon addition of ATP gamma S.