The delay in antibody production in response to infection with Treponema pallidum may be caused by a block in the differentiation of antigen-stimulated B (Bursa-dependent) lymphoid cells towards plasma cells. This hypothesis was tested by a study to detect clonal expansion of immunoglobulin-bearing B lymphoid cells by in-vitro immunofluorescence tests in patients with primary syphilis. In addition, antibodies eluted from circulating lymphoid cells were investigated for treponemal binding by the enzyme-linked immunosorbent assay, the T pallidum immobilisation test, and the immunoglobulin class-specific FTA-ABS test. Results indicated that the number of IgG-bearing lymphoid cells were increased in patients with primary syphilis. However, in only a few cases could antitreponemal antibodies be eluted from isolated lymphoid cells. For this reason, the original hypothesis was rejected.