Peripheral blood lymphocytes from dogs sensitized to streptolysin O (SLO) were assayed for migration inhibitory factor (MIF) production by the indirect MIF test, using guinea pig peritoneal exudate cells as the source of macrophages. A specific direct correlation was established between the degree of inhibition of migration and the concentration of SLO-stimulated supernatants from lymphocyte cultures (SLO-S) of untreated normal dogs. Undiluted SLO-S inhibited migration by 66.8%, whereas a dilution of 1:64 elicited a 3% inhibition. In parallel tests, purified protein derivative stimulation of lymphocytes from BCG-vaccinated dogs produced 92.6% inhibition. The effect of Corynebacterium parvum on SLO-specific MIF production was evaluated in three groups of dogs administered a single intramuscular injection of C. parvum at 5 or 50 mg/m(2) or 50 mg/m(2) in suspension with 10 mg of methylprednisolone. Inhibition of migration of macrophages exposed to a 1:4 dilution of SLO-S from dogs inoculated with C. parvum (5 mg/m(2)) was 33% greater (mean inhibition, 75%) than the same SLO-S dilution from uninoculated normal dogs (mean inhibition, 42%) (P < 0.0002). Similarly, lymphocytes from dogs administered 50 mg/m(2) caused an enhancement of migration inhibition, with a mean increase of 26% over controls (P < 0.002), whereas a dose of 50 mg/m(2) with methylprednisolone produced a 16% increase in migration inhibition (P < 0.05). The administration of C. parvum resulted in a three- to fourfold increase in the SLO-S dilution, which would reduce migration by 20% (MIF titer). This increase peaked between days 20 and 30 and lasted over 50 days post-C. parvum inoculation. These findings indicate that C. parvum specifically increases MIF production by canine lymphocytes in a linear correlation with SLO concentration and suggest its use as a stimulant of canine immunity.