The titer, immunoglobulin class, and staining pattern of smooth muscle antibodies (SMA) were assayed by indirect immunofluorescence in 15 patients with threatened abortion, 46 patients with spontaneous abortion, and 45 normal pregnant women in the first trimester to investigate possible abnormality of humoral immunity. An increased incidence of moderate and high titer SMA was found in both abnormal groups. SMA were mainly of the IgG class and gave a "vessel wall" staining pattern in immunofluorescence. This staining pattern suggests non-actin specificity of SMA. The more precise characterization of the antigen specificity of SMA in complications during pregnancy requires further investigation.