Four monoclonal antibodies, with P1 specificity were obtained after fusion of myeloma cells and spleen cells from mice immunized with turtle dove ovomucoid. Immediately after the fusion, the culture supernatants were studied for specificity with panels of erythrocytes and red blood cells sharing rare phenotypes (P1K, P2K, p) in the P system. After cloning, four monoclonal antibodies were produced, these antibodies strongly agglutinate P1 red blood cells, specially when they are used with 3% of dextran or with a 350 mmol/l concentration of sodium.