Serum and pleural effusion fluid were tested for CEA concentration in 83 advanced breast cancer patients, in 43 of whom CA 15-3 was also determined. All pleural effusions were clinically malignant. The sensitivity of the CEA test for the presence of pleural metastases was closer to that of the CA 15-3 test in effusion (0.59 and 0.79, respectively) than the sensitivity of CEA compared to CA 15-3 in serum (0.43 vs. 0.79). The use of two markers combined with cytology increased the diagnostic rate from 48% (cytologically positive) to 88% (cytologically positive and/or with one or both markers increased in effusion). A high diagnostic rate in cytologically negative effusions (65%), and in effusions presented as the sole metastatic involvement (100%), points to the clinical value of these two markers. Our results show that markers produced by pleural metastases may be secreted either into the effusion fluid or into serum, or both. This finding, as well as some other observations, are discussed in the present paper.