At disposal for tests were 156 blood sera collected from patients with acute myocardial infarction (AMI samples) and 67 blood sera collected from control persons. The sera were tested by the following reactions: complement fixation test (CFT) with genus-specific antigen (C. psittaci, Bio- veta, Ivanovice na Hané), ELISA in the immunoglobulin classes IgG and IgA using the MOMP (Vircell, Spain) and LPS (MEDAC, Hamburg, BRD) Chlamydia pneumoniae (C. p.) antigens, and indirect immunofluorescence test with MOMP C. p. antigen (MRL Diagnostics, USA). Eight AMI samples were not tested by CFT owing to haemolyis or lipaemia. CFT titres ranging from 1 : 4 to 1 : 16 were found in 28 of the 148 AMI samples (18.9 %) and 3 of the 67 control samples (4.5 %). The between-group difference was highly significant ( 2 = 7.795 at 0.01). Positive by rELISA anti LPS C. p. were 89 of the 155 AMI samples (57 %) and 22 of the 62 control samples (35 %). Also this difference was highly significant ( 2 = 8.528;  = 1 %). The following conclusions can be drawn from the results: 1. Chlamydia pneumoniae can play an important role in the aetiology of atherosclerosis leading to AMI. 2. rELISA anti-LPS can be recommended for routine serological diagnostics of C. pneumoniae and studies of its role in the aetiology of atherosclerosis. 3. The complement fixation test is still suitable for the screening of human chlamydiosis.

        Key words: Chlamydia pneumoniae - Myocardial infarction - Serological tests