Sera from 678 cows (all cows in each herd over 20 months of age) were tested for antibodies to M. paratuberculosis by the Johne's Absorbed EIA kit (Commonwealth Serum Laboratories, Parkville, Australia). Using the kit manufacturer's recommended cutoff we classified each cow as positive or negative. Dairy Herd Improvement records were obtained from each herd for the most recent lactation on each cow tested and transferred into a data base program. Comparison of milk production, mature equivalent 305 day lactation (ME 305), for the 43 test positive cows with their test negative herdmates revealed a 509 Kg (5.36%) lower rate of production for infected cows. The effect was minimal in lactations one and two but significant in lactation number three or greater. In addition to the reduction in milk quantity, the quality of milk from test positive cows was also less; lower protein ME 305 and lower fat ME 305. The net economic effect on productivity of cows increased with each lactation reaching over $200/test positive cow by lactation number three.