The kinetics of the antigen production of Chlamydia psittaci strains Izawa-1 and Pigeon-1041 (P-1041) was examined every 6 hr after infection up to 48 hr, by the indirect immunofluorescent antibody technique using monoclonal antibodies (MAbs). All three genus-specific antigenic determinants on lipopolysaccharide (LPS) appeared during the whole growth cycle. Antigenic determinants on proteins were, on the other hand, detected at various time periods from the early to the late stages of infection. However, a cross-reactive antigenic determinant on protein recognized by a MAb 3E9 was also detected during the whole growth cycle, similar to that on LPS. The time of appearance of common antigenic determinants on proteins of Izawa-1 and P-1041 was examined using cross-reactive MAbs, and it varied depending on heterologous and homologous MAbs. From the relationship between the detection of antigenic determinants and the morphological changes of chlamydial particles revealed by electron microscopy during the growth cycle, the antigenic determinants on proteins of Chlamydia psittaci were divided into two groups; one was specific to the elementary body and the other was coexisting in both the elementary body and the reticulate body.