Immunoglobulins obtained from bovine colostrum were heat treated at 70, 75, 80 and 85°C for 0-90 min. An ELISA was developed to determine the susceptibility of lactococci to bind heat treated IgG, IgM and IgA. IgA was the most heat sensitive of the immunoglobulins. Treatment at 80°C for 25 min completely inactivated IgA as evidenced by the inability of lactococci to bind IgA. IgM was completely inactivated upon heating to 85°C for 20 min whereas IgG was completely inactivated at 85°C after 30 min. Laser Scanning Microscopy verified results with ELISA. Severe thermal treatment is needed to reduce immunogenicity of bovine milk to prevent lactococcal agglutination.