T cell clones can present antigen in the absence of professional antigen presenting cells. T cells were stimulated with Ag peptide in conditions with T cells alone or T cells and B cells mixed together in culture. Addition of peptide to T cell cultures lacking B cells still induced (A) proliferation and (B) IFN-γ secretion in the responding T cells at levels not significantly different from T cell cultures containing B cells (GEE p value >0.05). Proliferation was measured by 3H incorporation and the results are expressed as a stimulation index (counts in stimulated condition/counts in unstimulated condition). IFN-γ secretion was measured by intracellular staining for IFN-γ expression after stimulation in the presence of brefeldin A. Data are compiled from five proliferation experiments and three intracellular cytokine staining experiments. Error bars signify SEM.