Chickens infected with infectious laryngotracheitis virus (ILTV) responded by producing virus-specific IgG in their sera, which increased steadily in concentration, but with slight fluctuations, until peak titres were reached 40 days post-inoculation (pi), immediately prior to the second challenge. Thereafter, following an initial lag, concentrations continued to increase for 21 days before falling slightly at the end of the experiment. In contrast, peak concentrations of ILTV-specific IgM were reached 6 days pi falling to their lowest levels by day 16, before increasing to a second peak and trough on days 26 and 32, respectively. This cyclical production of ILTV-specific IgM was confirmed in a second experiment. The pattern of production of ILTV-specific IgG, IgM and IgA, detected in tracheal washings, occurred in the same cyclical manner. IgM was produced first, peak concentrations being detected 5 days pi, whereas IgG and IgA did not peak until 10 days pi, with second peaks of each class being detected 25-30 days pi. The possibility that the cyclical antibody class response to ILTV infection is related to the previously reported intermittent pattern of re-excretion of the virus is discussed.