We have cloned a human counterpart to a guinea pig STE20-like kinase cDNA, designated human SLK (hSLK), from a human lung carcinomatous cell line A549 cDNA library. hSLK cDNA encodes a novel 1204 amino acid serine/threonine kinase for which the kinase domain located at the N-terminus shares considerable homology to that of the STE20-like kinase family. The C-terminal domain of hSLK includes both the coiled-coil structure and four Pro/Glu/Ser/Thr-rich (PEST) sequences, but not the GTPase-binding domain (GBD) that is characteristic of the p21-activated kinase (PAK) family, polyproline consensus binding sites, or the Leu-rich domain seen in the group I germinal center kinases (GCKs). Northern blot analysis indicated that hSLK was ubiquitously expressed. hSLK overexpressed in COS-7 cells phosphorylates itself as well as myelin basic protein used as a substrate. On the other hand, hSLK cannot activate any of the three well-characterized mitogen-activated protein kinase MAPK (ERK, JNK/SAPK and p38) pathways. Moreover, hSLK kinase activity is not upregulated by constitutive active forms of GTPases (RasV12, RacV12 and Cdc42V12). These structural and functional properties indicate that hSLK should be considered to be a new member of group II GCKs.