Day-old commercial layer chicks with maternally derived antibody to infectious bursal disease (IBD) were reared in four pens on a commercial farm, and chicks from the same flock were reared at the laboratory and examined for the decay of the maternal antibody and the development of susceptibility to IBD. The chicks in the pens on the farm were vaccinated with different combinations of commercial inactivated and live vaccines against IBD, transferred to the laboratory, and challenged with virulent IBD virus to assess the efficacy of the vaccination programmes. The chicks from one pen, given a live IBD vaccine only, did not develop antibodies and were fully susceptible to challenge by seven weeks old; only one to two weeks later than the unvaccinated control chicks reared at the laboratory. The chicks in the other three pens were inoculated with an inactivated vaccine, and the chicks in two of these pens were inoculated later with a live vaccine. All these chicks developed IBD antibodies and were protected by between 91 and 100 per cent against challenge. There was no demonstrable benefit in administering a live vaccine either alone or in addition to an inactivated oil emulsion vaccine.