The concentrations of the tumour markers CA 125 and CA 19-9 were determined in peritoneal, cyst and amniotic fluids, with particular attention being paid to certain reliability criteria of the assay methods. The antigens were measured in undiluted samples and after several dilutions. A recovery test was also performed and protein content evaluated. The results show high levels of CA 125 in all fluids; in descending order of concentration: amniotic (2376-3891 U ml-1), peritoneal (379-4040 U ml-1) and cyst fluid (124-466 U ml-1). Amniotic, peritoneal and cyst fluid concentrations of CA 19-9 were found to be 314-1008 U ml-1, 26.7-2182 U ml-1 and 226-2988 U ml-1, respectively. Recovery was between 80 and 100% for all fluids. CA 125 was easily assayable in all fluids, except amniotic and peritoneal which required dilution even of the samples which fell within the range of the standard curve before dilution. The presence of CA 125 and CA 19-9 in amniotic and cyst fluids emphasizes the non-specificity of these molecules and suggests caution in the interpretation of the results.