Analysing the large groups of sera and synovial fluids from patients with arthritis we observed, that frequency of the aCl presence in above mentioned materials was relatively high (about 30-50%) – very similar to aCl frequency in the sera of patients with autoimmune disease and we consider this as a result of the presence in synovial fluid phosphate – rich bacterial cell wall chemical compounds like LPS, teichoic acid or peptidoglycan – considered as „arthritogenic” factors. The aim of the study was to proove that aCl antibodies cross-reacting with LPS (endotoxin) from arthritic synovial fluids are part of the aPL antibodies pool and correlated with presence in joints bacterial cell wall components and beside they may be usefull in differential diagnosis of the different kind of arthritis and also in explanation of their aetiology at all. In prevalence of the 43 analysed synovial fluids antibodies to LPS were present in 86% of cases, aCl in 53.5% and endotoxin in about 30% and probably in additional 30% with presence inhibiting LAL test substances have been foud. Additionally in 3/4 of the analysed synovial fluids the appearance of anti LPS and aCl antibodies was strongly correlated. In about 75% of sera and in 50% of synovial fluids cases LPS was able to neutralize the ELISA-anti Cl test and „affinity” purified antibodies to LPS and Cl cross-reacting with heterologic antigen (aCl with LPS) – and vice versa/ in ELISA tests. The presence of the cross-reacting with LPS, aCl antibodies could be a remnant of the presence in joints bacterial arthritogenic compounds considered as aetiologic agents able to generating different kind of arthritis – especially when they are associated with the presence in the synovial fluids circulating immune complexes.