Antibodies to Mycobacterium avium subsp. paratuberculosis in milk samples from 6955 cows in 108 Danish herds were tested with enzyme-linked immunosorbent assay (ELISA). Optical density (OD) recorded on a continuous scale were standardized according to parity and stage of lactation as previous studies on the same material show significant increase in ELISA response with increasing parity and in the first and last weeks of the lactation of infected cows. A hierarchical three-level test day model was set up with Energy Corrected Test Day Milk yield (ECTM) as response. Besides standardized OD-values (stOD) - seven fixed covariates were included in the model together with quadratic terms and first order interactions. Cow and cow nested in herd were included as random effects. The material was analysed in three separate parity groups: Parity one, parity two and higher parities. Additionally the total milk production 305 DIM was analysed with an equivalent two-level model with Herd-ID as random effect. The slope of lactation curves after peak yield was significantly steeper in first and second parity cows, where an increase of one stOD-unit resulted in a 3.8 kg depression of ECTM day 305 in first parity and 3.3 kg in second parity. The correlation was linear, so halving the increase in stOD resulted in half the loss. The peak yield was not significantly affected by stOD, though close to in second parity (P<0.10). In third and older parities there was a significant effect of the quadratic term of stOD of -3.2 kg indicating exponentially increasing losses with increasing OD-values. The total production loss 0-305 DIM due to an increase of one stOD-unit was estimated to be 496 kg Energy Corrected Milk (ECM) in primiparous cows, 1318 kg ECM in second parity and 625 kg ECM in later parities.