The concentration of the tumor marker CA 125 in tumor tissue, cyst fluid, ascites fluid, and serum from patients with epithelial ovarian tumors was quantitated. Immunohistologic studies showed that CA 125 was present in 90% of the nonmucinous epithelial ovarian tumors. Quantitative analysis of the fluid from 57 cysts revealed that CA 125 was present in concentrations of up to 2140,000 U/ml in samples from malignant nonmucinous epithelial ovarian lesions, and up to 116,000 U/ml in mucinous tumors, but also in concentrations of up to 371,000 U/ml in benign serous cystadenomas. In contrast, pre-operative serum CA 125 levels were elevated in almost all of the patients with malignant ovarian tumors but not in most of those with benign ovarian tumors. These findings suggest that in benign ovarian tumors there is an effective barrier between the cyst fluid and the circulation that prevents the appearance of CA 125 in the serum, whereas in malignant tumors infiltrative growth leads to the release of antigen into the circulation. Furthermore, CA 125 values in ascites fluids were up to 130 times higher than the serum antigen levels, which indicates that the peritoneum serves as a barrier for high molecular weight tumor antigens. The current results show that tumor basement membranes and peritoneal barriers play a notable role in the transit of tumor antigens, one which must be taken into account in the monitoring of serum marker levels of cancer patients.