The purpose of this study was to estimate the genetic variation and the heritability of the ability to establish an immune response by producing antibodies to Mycobacterium avium subsp. paratuberculosis. Antibody levels were determined using an ELISA and measuring optical density (OD) values from milk samples of 11,535 cows from 99 herds. The pedigree of the 11,535 cows and information about days in milk, parity, milk yield, and others were obtained from the Danish Cattle database. The statistical analyses were made using a bivariate mixed animal model. The bivariate model with daily milk yield and OD as dependent variables showed a significant heritability of the ability to produce Mycobacterium avium subsp. paratuberculosis antibodies of 0.102 (genetic variance = 0.054) and a nonsignificant genetic correlation of −0.037 between daily milk yield and OD. When a sire model was used, the estimated heritability was 0.091. To evaluate whether intrauterine infection could be reflected in the OD, variation among 2715 dam-daughter or maternal sister groups was also estimated. Variation in this data was nonsignificant, possibly because only very few clinical cases or end-stage cows, i.e., heavy bacterial shedders may have been represented among the cows sampled. It does not appear that intrauterine transmission is of major importance in the transmission of paratuberculosis