We investigated the marker profile of human ascitic and cultured mesothelial 
cells, and compared it to that of ovarian carcinoma cells which are related in 
terms of their histogenesis, unrelated colon carcinomas being used as reference. 
Mesothelial and ovarian carcinoma cells could not be distinguished by 
(intermediate) filament typing, using monoclonal antibodies (MAbs) to keratins, 
vimentins and desmins. Colon carcinomas differed from mesothelial cells and 
ovarian carcinomas by the absence of keratin-7 filaments. The epithelial marker 
BW 495/36 was completely negative on mesothelial cells and positive on all 
ovarian and colon carcinoma cells. While CEA was found on about 85% of all colon 
carcinomas, CEA expression on mesothelial cells and ovarian carcinoma cells was 
below 20%. The ovarian carcinoma markers (OV-TL 3, OV-TL 10, OC 125, MOV 18) 
were strongly positive on ovarian carcinomas and negative on colon carcinomas 
(or limited to traces of immunofluorescence on some samples). Although the 
mesothelial cells showed weak or negative reactivity with these markers, OC 125 
antigen was found by immunoelectron microscopy on the surface of cultured 
mesothelial cells, and was shed in the culture supernatant at concentrations of 
50, 28, and 25 CA 125 U/ml/10(4) positive cells. This suggests that mesothelial 
cells may be responsible for the synthesis of CA 125 in ascitic fluid. The data 
indicate that ovarian carcinomas, mesothelial cells and colon carcinomas can be 
distinguished using a combination of anti-keratin antibodies with BW 495/36 and 
anti-ovarian carcinoma markers.