Abstract: This article reports the analysis of 150 patients with positive anti-RNP antibody test detected among 785 patients in 5 hospitals by passive hemagglutination test combined with counterimmuno-electrophoresis with calf thymus extract (ENA). The positive rate of ant-RNP in MCTD was the highest being 100%. RNP antibody could also be found in other connective tissue diseases, occasionally in non-cosmective tissue diseases, but the titer was low. High titer of anti-RNP antibody was helpful for the diagnosis of MCTD, and regarded also to be the serological marker of this disease. The anti-RNP antibody proved to be correlated to certain clinical manifestations, also possibly related to the pathogenesis of the disease. From this study, the authors consider MCTD is a distinct disease entity with a characteristic clinical manifestations and immunologic abnormalities. The diagnostic criteria of MCTD proposed by Sharp (1983) was practical and worth while to be used and further investigated.