Mitogen-activated protein (MAP) kinases are intracellular serine/threonine kinases activated by dual phosphorylation of adjacent threonine (T) and tyrosine (Y). A diverse number of extracellular signals induce activation of MAP kinases. Here we describe the cloning of a cDNA encoding human p38 MAP kinase (p38). The amino acid sequence of human p38 is 99.4% identical to mouse p38 [Han et al. (1994) Science 265, 808-11]. Like murine p38, the dual phosphorylation site of human p38 MAP kinase is characterized by a TGY sequence. Previous studies have described activation of p38 following exposure to products of microbial pathogens, physical-chemical stimuli and cytokines. The highly conserved nature of p38 suggests the importance of its function in regulating cellular responses.