One-day-old chickens with maternally derived antibodies to infectious bronchitis virus (IBV) were vaccinated against IB by eye-drop. At 2 and 3 weeks of age the synthesis of immunoglobulins of the IgA isotype in the Harderian gland was demonstrated by immunofluorescence. With an anti IgG conjugate diffuse fluorescence of IgG was observed in the gland at 2, 3 and 4 weeks of age. Involvement of the Harderian gland in the synthesis or the secretion of IgM after IB vaccination could not be demonstrated by immunofluorescence. A quantitative estimation of immunoglobulins was carried out by laser nephelometry. At 2 weeks of age the concentration of antibodies against IBV of the IgA isotype (IgA-IBV) was higher in the tears than in the serum. This and the positive fluorescence indicate a local synthesis of IgA-IBV in the Harderian gland. The highest concentration of IgG antibodies against IBV (IgG-IBV) in the serum was measured at 2 weeks of age, when IgG antibodies could not be detected in the tears. In the tears the IgG-IBV concentration increased from 2 weeks up to and including 5 weeks of age, when the concentration in the tears was higher than in the serum. These findings and the diffuse IgG fluorescence in the Harderian gland suggest a mainly systemic production of IgG-IBV and an active and selective transport of IgG-IBV from the serum to the tears by the Harderian gland. Challenge at 6 weeks of age of the vaccinated chicks caused a sharp increase of IgG-IBV in the serum and a decrease of IgA-IBV in the tears 1 week later. Challenge of the unvaccinated control chicks resulted in a distinct rise of IgA-IBV in the tears and in a low IgG-IBV concentration in the serum after 1 week. These findings and the neutralisation indices measured for sera collected at 5 and 7 weeks are discussed.