A method was developed to detect individual interferon-gamma (IFN-gamma)- and interleukin-5 (IL-5)-producing cells among freshly isolated T cell populations and long term lines of CD4+ Peyer's patch T cells using frozen semi-thin sections of paraformaldehyde fixed-T cells and immunofluorescence techniques. Using this method, individual CD4+ Peyer's patch T cells could be shown to produce IFN-gamma, characteristic of the T helper 1 (Th1) T cell type, IL-5, characteristic of the T helper 2 (Th2) T cell type, as well as both IFN-gamma and IL-5. These data support the notion that Th1 and Th2 cells derive from a common T cell precursor.