A sensitive ELISA is described which allows the detection and quantitation of immunoglobulin positive Peyer's patch and spleen lymphocytes in tissue culture microtitre plates. A methanol fixation step is used to fix the cells to the microtitre plate. The effects of various mitogens and an antigen on the number of immunoglobulin positive cells present after a 7 day culture were studied. Results were expressed either in optical densities or as percentages of the unstimulated control. The method proves a useful addition to existing techniques for studying the effects of mitogens and other agents on in vitro immunoglobulin synthesis by lymphocytes, and reflects immunoglobulin levels in the culture supernatants.