There is convincing epidemiological evidence that multiparity provides protection against the development of breast cancer. In the present study we evaluated the levels of MUC1 and MUC1 circulating immune complexes (MUC1-CIC) in 135 serum samples obtained from healthy women. The study population included 13 women who had never been pregnant, 31 primiparous pregnant women, 36 multiparous pregnant women who had lactated, 5 multiparous pregnant women who had never lactated, 24 multiparous non-pregnant women who were lactating at the time of the study, 24 multiparous non-pregnant women who had lactated, and 2 multiparous non-pregnant women who had never lactated. The purpose of this work was to detect MUC1 variations during pregnancy and lactation as well as to study the possible induction of a humoral immune response against MUC1 in these conditions. We employed ELISA techniques to measure MUC1 (CASA test) and MUC1-CIC (IgM and IgG) using two anti-MUC1 monoclonal antibodies (MAbs): C595 and SM3. Statistical analysis was performed using the ANOVA test. The pooled results pertaining to pregnant versus non-pregnant women were compared and significant differences were observed in MUC1 and MUC1-CIC-lgM levels detected with both MAbs; the MUC1-CIC-lgG levels detected with C595 were increased in the pregnant group while the MUC1-CIC-lgG levels detected with SM3 did not show any significant differences. When the results were compared between lactating and non-lactating women, no significant differences were found. In conclusion, MUC1 and MUC1-CIC-lgM, detected with both MAbs, and MUC1-CIC-4gG levels detected with the MAb C595 are apparently induced by pregnancy.