Anticardiolipin (aCL) antibodies were assessed in isotypes IgG, IgM and IgA by the enzyme immunochemical technique in serum of 86 subjects with diffuse connective tissue affections and in 75 subjects of three control groups (syphilis, syndrome of common variable immunodeficiency and blood donors). In systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and syphilis the mean values of the three isotypes of aCL antibodies were significantly higher than in blood donors (p = 0.05 to 0.001); in diffuse scleroderma and primary polymyositis/dermatomyositis in isotype IgG (p = 0.01-0.001). Positive findings of aCL antibodies (isolated or in combinations of Ig isotypes (were found most frequently in SLE (34.4%), RA (33.3%) and syphilis (66.6%); sera of blood donors were positive in 8.7%. Venous thrombosis was recorded in the case-records of 28% patients with SLE but only in 5.4% of those with RA. Spontaneous abortion terminated 8/66 pregnancies in 28 women with SLE. In one female patient with SLE the aCL syndrome was detected. On account of frequent positivity of aCL antibodies in syphilis, the authors consider it essential to rule out the coincidence of this disease. Examination of aCL-IgA antibodies extends the detection of positive cases (isolated or in combinations of Ig) in SLE and RA.