The phenotype of milk-derived and peripheral blood lymphocytes from normal and coeliac subjects was assessed for CD3, alpha beta-TcR (T cell receptor), gamma delta-TcR, CD4, CD8, HML-1 (human mucosal lymphocyte) determinants, and activation was measured by interleukin-2 receptor (IL-2R) expression. Milk cells from normal and coeliac subjects were analysed by manual immunofluorescence and milk and blood cells from normal subjects were analysed by flow cytometry. Milk cells from two coeliac subjects were tested for proliferation to gluten antigen. The CD4:CD8 ratio of milk lymphocytes from both normal and coeliac subjects was similar (0.78-1.1), but lower than that present in blood (1.5-2.1). The IL-2R expression of milk lymphocytes from both coeliac and normal subjects was increased by 3-6 times compared with peripheral blood cells. For example, IL-2R was present on 27.3% of milk CD3+ lymphocytes and on 8.0% of blood CD3+ lymphocytes from normal subjects. gamma delta- and HML-1+ T cells were increased 4.2-fold and 12-fold respectively compared with blood lymphocytes. Milk cells from coeliac subjects showed specific proliferation to gluten but not to soya bean antigen. We conclude that milk cells have a 'mucosal' phenotype, with increased gamma delta-TcR, CD8+ and HML-1+ T cells, and have an increased proportion of activated cells. Milk cells from coeliac subjects have no 'toxic' phenotype, but show functional reactivity by specific proliferation to gluten antigen.