A total of 443 Large White pigs (castrated males, dam line) tested for performance traits in a pig test station (UE450, INRA, Le Rheu, France) was included in the study. The pigs were distributed in seven contemporary groups and belonged to 307 nuclear families obtained from 106 boars, with an average of 4.1 (+/2 1.7) piglets per boar. Animals were born and weaned in 16 different selection herds and arrived in the test station at five weeks of age with no prior vaccination. They were placed into pens of 30 piglets in a post weaning unit and vaccinated against M. hyopneumoniae (Stellamune, Pfizer, one injection) one day after their arrival in the test station, when 36.3 days old in average. All pigs were apparently healthy with no clinical sign of infection. All animals were sampled three weeks after vaccination. Blood samples were collected via the external jugular vein into tubes with or without anti-coagulants, according to further use.