Laminin components were solubilized from basement membranes of amnion, chorion and chorionic microvessels of human placenta without prior protease digestion. These structures, after isolation, were initially processed in a sonicator bath containing a solution of Triton X-100, EDTA and 2M NaCl and the laminins extracted sequentially with 0.5M NaCl, 8 M urea, and 8 M urea + 2% 2-mercaptoethanol + 2% SDS. A high molecular weight (appr. 1 x 10(6)) complex containing laminins was purified by gel filtration on a Sepharose CL-2B column. This complex migrated as a single band on gel electrophoresis before reduction but resolved, after reduction, into four major laminin components, laminin A (350,000 M.W.), laminin M (240,000 M.W.) and laminins B1 and B2 (195,000 and 185,000 M.W.). Laminin M is a new molecular species of this protein.