IgG antiphospholipid antibodies binding to phosphatidylethanolamine (PE). When lamellar PE (from bovine brain) was used as the solid-phase antigen in ELISA, only 5 of 59 plasma samples had an absorbance exceeding 2 SD the mean of controls. When hexagonal II PE (Staclot LA) was used as the solid phase antigen, only two samples reacted. When the solid-phase antigen in ELISA plates was represented by the calcium-mediated complex of hexagonal II PE and human prothrombin, 41 samples (69%) had an absorbance exceeding 2 SD the mean of controls. Horizontal lines represent the upper limit of 20 normal controls (ie, mean + 2 SD). *Values represent the mean ± SD of patients’ group.