Abnormal forms of Chlamydia trachomatis have been induced in vitro by a variety of methods including nutrient deprivation, addition of cytokines and addition of antibiotics. These forms have been shown to have altered morphology and infectivity and have been implicated in persistent infections in vivo although there is little direct evidence for their presence. Likely sites for abnormal forms in vivo are the genital tract and the synovial tissue of reactive arthritis patients, and T cells isolated from the synovial tissue have been shown to be specific for chlamydial antigens, in particular the Hsp60. Since T cell specificity is so important in reactive arthritis disease the antigenic composition of abnormal forms induced by Interferon-gamma and amino acid deprivation has been examined by western blotting in two strains ofC. trachomatis belonging to different biovars. The degree of abnormality of the organisms was found to increase as the treatments became more severe. No simple patterns of antigenic changes were found and differences in the antigenic composition were seen in abnormal forms induced by the different treatments and also in the different strains.