Ca 15.3 is a tumor marker used for breast carcinoma, since one epitope is an antigen present in milk fat globules. Serum from 171 patients with breast cancer upon initial presentation was studied for Ca 15.3. In the first 72 cases, the authors compared RIA vs. ELISA using a simple linear regression. On the following 99, only ELISA was performed. With all 171 patients, a clinical association between Ca 15.3 measurement and age, stage and hormone receptors was carried out. Correlation coefficient between RIA and ELISA was 0.85. Of 104 patients below 50 years of age, 88 had normal Ca 15.3 and 16, elevated; 67 were older than 50 years, 46 had normal Ca 15.3 and 21, elevated (p=0.022). Ca 15.3 was elevated in 11% of patients with clinical stages I/II, and 89% in stages III/IV (p=0.0001). The association of Ca 15.3 with hormone receptors was not significant. In conclusion, ELISA and RIA measure Ca 15.3 with comparable results, the first method has the advantage of not using radioactivity. The authors found higher probability of elevated Ca 15.3 in older patients and in those with advanced disease.