In order to study the activity of immunocompetent lymphocytes in normal human adult mucosal lymphoid tissues in the gut, viable mononuclear cells (MNC) from surgically removed appendices were isolated with a rapid and improved technique, and their immune reactivity determined in vitro. Nearly two-thirds of the isolated cells were B cells, composed of sIgM (30%), sIgA (19%) and sIgG (12%), less than one-third of cells being T cells, composed of OKT4+ helper/inducer T cells (18%) and OKT8+ suppressor/cytotoxic T cells (9%). Isolated B cells were responsive to SAC (Cowan type I Staphylococcus aureus) with marked augmentation of IgG production, but not to lipopolysaccharides (LPS). In the presence or absence of pokeweed mitogen (PWM) and protein A, all isotype-specific Ig production was augmented along with increasing ratios of autologous T cells to B cells, whereas in the presence of anti-HLA-DR antibodies the production of IgA and IgM was reduced to a great extent. Finally, in vitro induction of concanavalin A (Con A)-induced suppressor T (Ts) cells was less than that of helper T (Th) cells. Thus, normal adult appendiceal (Ap) lymphocyte reactivity is predominated by helper T (Th) cells; IgA isotype-specific Ig production appears to be largely regulated by the autoreactive Th subset. This responsiveness to exogenous stimuli may play an important role in the gut mucosal immune responsiveness.