We previously demonstrated that glia maturation factor (GMF), a 17-kDa brain protein, can be phosphorylated in test tube by several protein kinases, and that endogenous GMF is rapidly phosphorylated upon stimulation of astrocytes by phorbol 12-myristate 13-acetate. We further observed that protein kinase A (PKA)-phosphorylated GMF is a potent inhibitor (IC50 = 3 nM) of the ERK1/ERK2 (p44/p42) subfamily of mitogen-activated protein (MAP) kinase. We now report that, by contrast, PKA-phosphorylated GMF strongly enhances the activity of a related but distinct subfamily of MAP kinase, the p38 MAP kinase, showing an increase of 60-fold over baseline and an EC50 of 7 nM. Non-phosphorylated GMF or GMF phosphorylated by other kinases exhibits only minimal effect. The intracellular interaction of PKA, GMF, and p38 is supported by the phosphorylation of GMF upon cellular stimulation by forskolin (blocked by PKA inhibitor) and by the co-immunoprecipitation of p38 with GMF from cell lysates. Withdrawal of nerve growth factor from PC12 leads to increased GMF phosphorylation with a time course similar to that reported for p38 activation. The results correlate well with a previous report that ERK and p38 carry out opposing functions and implicate GMF as a regulator of major cellular events.