Chicks at 2, 4 or 6 weeks of age were experimentally infected individually with a nephrosis/nephritis-causing avian infectious bronchitis virus (IBV) strain Kagoshima-34. The susceptibility of chicks in each group to the infection was compared, based on the clinical signs, excretion of virus in the faeces and antibody titres in the serum. The results showed that although all chicks appeared to be susceptible to IBV infection, the most severe clinical response was observed following infection at 2-week-old. Likewise, whilst the virus was also recovered from the faeces of all the chicks infected, the duration of viral excretion was longest in the 2-week-old chicks. A high antibody titre was detected at 4 weeks post infection (PI) and was maintained for at least another 16 weeks in the 4- and 6-week-old chicks. In contrast, a low antibody titre was detected only between 8 to 12 weeks PI in the 2-week-old chicks. Thereafter, no antibody was detected despite the presence of clinical signs.