Variation, and relation to pneumonia and mastitis, of four immunoglobulins, G1, G2, M, and A of blood serum measured by radial immunodiffusion, was studied in a sample of 190 Canadian Holstein-Friesian females sired by 22 bulls in artificial insemination. Regression analyses indicated that combined effects of age, stage of lactation, parity, and cow's sire's milk rating accounted for 6, 37, 2, and 3% of the total variation in G1, G2, M, and A. The G2 increased significantly with age, parity, and stage of lactation and decreased slightly with increasing sire milk ratings. Heritabilities by paternal half-sister correlation varied from 0 for G1 and A through .28 to .68 for IgG2 and .08 to .85 for IgM. Immunoglobulin G2 was positively correlated with clinical mastitis (.24) and incidence of enzootic pneumonia (.13) indicating further study of immunoglobulin G2 and its relationship to disease is warranted. No significant correlations existed between other immunoglobulins and mastitis or pneumonia, nor was a cow's production related to immunoglobulin. Selection to alter immunoglobulin content and reduce disease may be feasible and should be examined further.