It has been claimed that some consumers tolerate unhomogenized cow's milk better than homogenized cow's milk. The aim of this study was to compare the effect of homogenized and unhomogenized milk on the production of specific antibodies against cow's milk proteins (casein, beta-lactoglobulin) and bovine insulin. Thirty-six milk-tolerant volunteers were challenged with homogenized and unhomogenized cow's milk for 28 days in a randomized, open, cross-over study (>400 ml of milk daily). From serum samples taken at baseline and at the end of both milk challenges IgG and IgA were measured by ELISA against casein, beta-lactoglobulin and bovine insulin, and IgE against casein. The antibody production of the subjects remained constant for over 2 months, with no differences between the challenges with homogenized and unhomogenized milk. Inter-individual variation was greater than the intra-individual variation or the variation between the milk challenges. The order of the milk challenges did not affect the results. In conclusion, there seems to be no difference in the immunological responses to homogenized and unhomogenized milk in healthy adults with a good tolerance of milk.