The concentration of IgG and IgA was measured in the supernatants of peripheral blood mononuclear cells and of cells harvested from the intestinal lamina propria, which were cultured in vitro in the presence or absence of mitogens. The lamina propria mononuclear cells were harvested by collagenase digestion of macroscopically normal mucosa from 10 fresh surgical resections for carcinoma. Secretion of IgA in cultures of unstimulated lamina propria mononuclear cells greatly exceeded that of IgG. The addition of pokeweed mitogen increased Ig secretion by cultures of peripheral blood mononuclear cells but decreased Ig secretion by lamina propria mononuclear cells. The addition of concanavalin A suppressed Ig synthesis by pokeweed mitogen stimulated cells more in cultures of peripheral blood mononuclear cells than in lamina propria mononuclear cells. Cycloheximide inhibited Ig secretion by more than 90% in cultures of peripheral blood mononuclear cells, but there was less inhibition in cultures of lamina propria mononuclear cells. In the four unstimulated cultures of lamina propria mononuclear cells examined, over 75% of the Ig was secreted in the first three to four days of culture. The results indicate that lamina propria mononuclear cells are refractory to the inductive and suppressive signals of mitogens, and represent an activated cell population which is committed to Ig secretion before being cultured.