Abstract
Chlamydial group antigen was extracted from Chlamydia trachomatis strain SA2(f) and used as the antigen for an ELISA. The assay was reproducible since chlamydial antibody titres differed by no more than twofold when sera were tested on up to eight occasions. In tests on sera from 75 patients attending venereal disease or rheumatology clinics, the results of the ELISA and of a microimmunofluorescence (MIF) technique were similar for 61 of the sera, that is an 81% agreement. However, the ELISA was a little more sensitive than the MIF technique and at least tenfold more sensitive than the complement fixation procedure. Chlamydial IgG antibody at a titre of 1/greater than or equal to 16 was detected by the ELISA in 6% of children's sera, in 20% of sera from adult patients attending hospital with non-venereal diseases and in 85% of sera from persons attending venereal disease or rheumatology clinics. IgM and IgG antibodies were detected also by the ELISA in the sera of chimpanzees and marmosets which had been infected genitally with C trachomatis and, in general, the titres were greater than those recorded by the MIF test. The value of the ELISA in comparison with the MIF test is discussed.