The specificity and sensitivity of anti-SSA (Ro) antibody detection by double immunodiffusion, ELISA, western blot and a new method named CAPE blot were studied. Using 79 normal sera, 61 sera positive for anti-SSA (Ro) antibodies (double immunodiffusion), and 39 sera without anti-SSA (Ro) antibodies but containing anti-Sm, anti-RNP, anti-SSB (La), anti-Scl 70 and anti-PCNA antibodies, we compared ELISA, western blot and CAPE blot with the immunodiffusion assay. The sensitivities of the three methods were 100, 62 and 100%, respectively. Sera from 196 patients with systemic lupus erythematosus (SLE) were tested. The prevalence of anti-SSA (Ro) antibodies in this group was scored as 26% by double immunodiffusion, 47% by ELISA, 25% by western blot and 38% by CAPE blot.