For the serodiagnosis of syphilis a quantitative passive agglutination (MCA-TP) test for antibodies to Treponema pallidum was performed with chemically stable microcapsules with no antigenic activity instead of with conventional sheep erythrocytes. The microcapsules were easily sensitised with the antigen of sonicated Treponema pallidum by treatment with glutaraldehyde. Compared with the Treponema pallidum haemagglutination test (TPHA) the MCA-TP test was superior for detecting cases of primary syphilis. Furthermore, the decrease in antibody titre during treatment was more evident in this test than in the FTA-ABS or the TPHA tests. The MCA-TP test performed on IgM and IgG gel-filtered fractions of sera from patients with syphilis proved that the sensitised microcapsule antigen reacted sharply with the IgM antibodies specific to syphilis.