Two groups of 96 pigs were studied to determine the influence of weaning age, nursery site and a challenge to their immune system on their performance. The weaning ages were 11 to 16 days and 16 to 21 days. One nursery was on-site and the second nursery was off-site. Immune activation was stimulated by the administration of infectious bovine rhinotracheitis virus (IBR) vaccine to half of the pigs at each site. Serum virus neutralisation titres to IBR and total immunoglobulins were monitored in some of the pigs in each group. Performance was measured in terms of feed intake, average daily gain in weight, and feed conversion ratio (FCR). The mean serum immunoglobulin concentrations of all the groups of pigs tended to decrease in the first two weeks after weaning and then increase. Twelve of 20 pigs vaccinated with IBR had neutralisation titres to the virus. The site of the nursery did not significantly affect average daily gain in weight, feed intake or FCR. Pigs weaned at 16 to 21 days of age had a significantly better daily gain in weight than the pigs weaned at 11 to 16 days of age. Immune stimulation of the older weaned pigs did not influence their performance, but it had a significantly (P<0.016) negative effect on the performance of the younger weaned pigs.