The presence of soluble laminin fragments in urine of healthy subjects, patients with diabetes, and patients with tumours was studied using sandwich immunoenzymometric assay technique. The form of urinary laminin (ULN) fragments was dramatically different from that of intact laminin, so ULN could be detected only by using monoclonal antibodies. Mean levels of ULN in lung tumour were significantly higher (171 micrograms gram-1 creatinine) than those in healthy subjects, patients, with diabetes, patients with stomach tumour, and patients with colon tumour (respectively 91, 92, 77 and 53 micrograms gram-1 creatinine). Immunopurified ULN fragments showed an apparent molecular mass of 42 KD on electrophoresis. This fragment was recognised as being derived from the N-terminal region of laminin B2 chain, because the N-terminal residues of ULN were found to be completely homologous to B2 chain. These data suggested that ULN was almost all fragmented, consisted mainly of N-terminal domain of the B2 chain, and was suspected of a tumour-associated protein fragments probably derived from basement membrane degraded proteolytically by tumour cells. ULN, increased in tumour patients, could be a potential clinical marker for monitoring the turnover of basement membrane in tumours.