Mucosal T cells in explants of human fetal small intestine (17-20 weeks gestation) in organ culture were activated in situ using monoclonal anti-CD3 antibody. Changes in the distribution of T cells within the mucosa, and their phenotype, were monitored by immunohistochemistry on frozen sections. Anti-CD3 stimulated T cells (as determined by expression of CD25) were predominantly in the lamina propria and were rarely seen in the epithelium. In control cultures, after 72 h, CD3+ IEL decreased to low numbers compared to day zero. However, in cultures treated with anti-CD3, IEL numbers were maintained and in some experiments significantly increased compared to day zero levels. At onset of culture 50-60% of CD3+ IEL were CD4-, 8-, and virtually all were HML-1+. The T cell infiltrate into the epithelium induced by activation of lamina propria T cells with anti-CD3 was also mostly CD3+, 4-, 8-, HML-1+. These experiments provide strong evidence that increases in IEL numbers can be a consequence of lamina propria T cell activation.