Six dairy cows were immunized subcutaneously with purified type 5 capsular polysaccharide (CP5) of Staphylococcus aureus or CP5-ovalbumin conjugate in Freund's incomplete adjuvant. The CP5 antibodies elicited were measured in sera and analysed with regard to isotypes by enzyme-linked immunosorbent assay. At the doses tested, the purified CP5 did not induce a humoral response in the cows. Immunization of two cows with the CP5-ovalbumin conjugate elicited a CP5 antibody response mainly of the IgG2 isotype, which culminated 4 weeks later. A second injection of conjugate, 3 months after the first one, resulted in a rapid and durable anti-CP5 response without exceeding the first antibody peak value. Intramammary infusion of purified CP5 failed to provoke an inflammatory response in the milk of the immunized cows. In contrast, a marked recruitment of cells was recorded in the milk of the sensitized cows after intramammary infusion of ovalbumin. These results demonstrate that injection of CP5-protein carrier conjugate in cows entails both antibody response against CP5 and carrier-specific recruitment of cells in milk of immunized animals.