The difference in the functional activity of the isotypes A and B of component C4 of human complement was used to determine their ratio to detect the inherited deficiency of the isotypes. The frequency of deficiency in healthy persons blood donors was equal for C4A and C4B (0.14 for each isotype), i.e. 14% of total number (22) donors, or 28% totally. These results agree with the literary data, on which the frequency of deficiency of C4A is 0.14, and of C4B is 0.11-0.16. The inherent deficiencies of C4A and C4B for persons infected by Chlamydia were studied. For this purpose the patients (35 persons) with in blood antibodies (IgG or IgM) to Chlamydia (C. trachomatis, C. psittaci and C. pneumoniae) were investigated. The frequencies of deficiency of C4A and C4B were 0.29 and 0.46 respectively. Thus, the number of the undeficiency patients was only 25%, while among healthy persons 70-75% of individuals not having deficiencies of isotypes C4 were observed. The deficiencies of isotypes of C4 at this pathology is detected for the first time. The obtained data suggest the existence of the predisposition to the development of diseases stipulated by Chlamydia in persons with inherent deficiency of C4 component of complement.