The effect of monocytes as an activator of a function of lymphocytes in pigs was studied using an in vitro Ig production system with pokeweed mitogen (PWM). When mononuclear cells (lymphocytes) from adult pigs were stimulated by PWM, optimal number of adherent cells (monocytes) collected from serum coated plastic Petri dishes were required for the generation of Ig producing cells, whereas the production was suppressed by the addition of excessive adherent cells. In neonatal and suckling piglets up to 6 weeks of age, PWM induced B lymphocyte differentiation was enhanced by addition of adherent cells from peripheral blood of adult pigs. On the other hand, adherent cells derived from piglets did not have the effect of increasing Ig production in the in vitro Ig production system with lymphocytes (non-adherent) derived from adult pigs
