The aim of the study was to show the presence of endotoxin in the synovial fluids from the patients suffering of Ctd-s (in prevalence RA and other undifferentiated joint arthritis). The usefulness of the applied LAL-test (Limulus Amebocyte Lysate) was evaluated (just in endotoxin /LPS/ assessment), as a main and functionally most important of the Gram-negative bacterial cell wall component. It was proved, that in the group of 50 synovial fluids from Ctd-s patients 42% was LAL-positive (with presence of endotoxin) and in 60% of RA patients LAL-test was also positive. Simultaneously, in the total amount of LAL-negative synovial fluids we found, that in 66.6% the inhibiting LAL-test substances (including antibody) have been found. It suggest, that in tested material we have obtained considerable amount of the fals-negative tests (results), because that a retrospective analysis showed in tested synovial fluids the presence of anti-LPS or antibodies to selected LPS subcomponents up to 80%. In discussion a possible influence of endotoxin, especially bacilli from genus Salmonella enteritidis (or S. typhimurium) and their toxic component (lipid A) on the induction acute phase Ctd-s (especially in RA and undifferentiated inflammation of joints) – basing on our own experiments and literature, was touched.