We have identified a second human homology of the yeast Ste20 protein kinase family, which we designate MST2. MST2 is most similar to the previously identified MST1 protein kinase (78% identity, 88% similarity). Northern analysis indicates that MST2 mRNA is expressed at high levels in adult kidney, skeletal and placental tissues and at very low levels in adult heart, lung, liver and brain tissues. An in vitro kinase assay indicates that MST2 can phosphorylate an exogenous substrate, as well as itself, and phospho-amino-acid analysis indicates that it is a serine/threonine protein kinase. The identification of MST2 suggests that there may be subfamilies of Ste20-like protein kinases and that MST1 and MST2 may define one of these subfamilies.