Abstract Multiple serine/threonine kinases were detected in a bone marrow stromal cell line. One of these, the murin a ctivin receptor l ike k inase-1 (ALK-1) homolog was cloned and sequenced. The expressed recombinant protein (62 kDa) was consistent in size with that predicted by the cDNA (58.6 kDa). On Western blots, a goat polyclonal antibody detected the native ALK-1 protein in bone marrow stromal cells, lung, brain, kidney and spleen. Two protein species of 60 kDa and 72-76 kDa were detected. Glycosylation events or alternative splicing may account for the larger protein specie.