The antibodies to ribonucleoprotein (RNP) and Sm antigen were studied in patients with various connective tissue diseases including systemic lupus erythematosus (SLE) (150), scleroderma (28), polymyositis (25), rheumatoid arthritis (25), mixed connective tissue diseases (MCTD) (29), sclerodermatomysitis (8), sicca complex (70) and others (2). One hundred fifty patients with SLE were divided into three groups: group 1 patients (37) had anti-RNP alone, group 2 patients (46) had anti-RNP and anti-Sm and group 3 patients (67) had neither anti-RNP nor anti-Sm. Raynaud's phenomenon and central nervovs disease were significantly more common in patients of groups 1 and 2 than group 3. Renal disease, elevated n-DNA binding and decreased CH50 were less common in patients of group 1 than group 3. LE cell phenomenon was less common in groups 1 and 2 than group 3. Renal disease was significantly less common in group 1 than group 2. Scleroderma patients with anti-RNP had fever, elevated ESR and hypergammaglobulinemia more frequently than those without anti-RNP. Twenty-nine patients with MCTD were divided into 2 groups: 16 patients with definite or probable collagen disease and 13 patients with probable collagen disease. The patients of the second group had swollen hands more frequently, and usually only had anti-RNP. The serological and clinical features of patients in the first group appeared to be more uniform and distinctive than those of patients in the second group. Patients with sicca complex and anti-RNP showed a significantly higher frequency of fever, arthralgia, Raynaud's phenomenon, erythema and lower frequency of the rheumatoid factor than patients without anti-RNP.