Ig production by splenic human B cells that express different surface Ig isotypes were analysed in limiting dilution cultures. Therefore, FACS sorted IgM+, IgG+ and IgA1+ B cells were stimulated with PMA-activated EL4 thymoma cells as helper cells in the presence of IL-2 and IL-4. We found that at least every second B cell responded in vitro and secreted the antibody corresponding to its surface Ig isotype. IgE secreting cells developed from surface Igm+ D+ cells (1 31 to 1 167), but not from IgG+ or IgA1+ cells (⪡ 1 5000). Negative signalling of the IgM+ B cells by addition of anti-IgM antibodies into the cultures reduced the number of single IgM producing cells by> 85%, and completely inhibited IgE switch. In contrast, anti-IgG and anti-IgA antibodies did not reduce the IgE response. The results indicate a direct switch from IgM to IgE secretion in vitro. In contrast to IgE, IgA secreting cells developed from IgM+ D+(1 30 to 1 51) and from IgG+ B cells (1 14 to 1 25). Negative signalling of the IgG+ B cell subset within total B cells by anti-IgG antibodies suppressed the development of IgG as well as IgA producing cells, but did not inhibit IgM and IgE responses. This indicates a sequential switch from IgM via IgG to IgA. Taken together, this study indicates that IgE secreting cells are derived directly from IgM+ D+ B cells by non-sequential switching, whereas IgA producing cells preferentially develop by sequential switching via IgG+ B cells.