AIMS--To investigate the secretion of the tumour marker CA 125 by cultured human mesothelial cells; to determine if secretion of CA 125 could be observed by activating the mesothelial monolayers with different cytokines. METHODS--Mesothelial cells were isolated from human omentum and cultured to confluent monolayers on perforated polycarbonate membranes (pore size 0.4 micron). The mesothelial monolayers were activated and the apical and basolateral secretion of CA 125 compared. To investigate the influence of cytokines, mesothelial cells were cultured and activated with recombinant interleukin-1 beta (rIL-1 beta), tumour necrosis factor-alpha (TNF-alpha) or lipopolysaccharide from Escherichia coli. The secretion of CA 125 was tested using a microparticle enzyme immunoassay. RESULTS--Mesothelial monolayers secreted CA 125 in a polarised manner with preference for the apical side. Apical polarisation occurred irrespective of the side of the inducing stimulus (p < or = 0.05). Non-activated cultured mesothelial monolayers secreted significant quantities of CA 125, indicating constitutive production of this protein. However, CA 125 production was significantly enhanced if mesothelial cells were incubated with rIL-1 beta (p < or = 0.05), TNF-alpha (p < or = 0.05), and E coli LPS (p < or = 0.01). CONCLUSIONS--Human mesothelial monolayers secrete CA 125 preferentially from their apical surfaces. The secretion of CA 125 can be enhanced by the inflammatory cytokines Il-1 beta, TNF-alpha, and by E coli LPS.