Vasoactive intestinal peptide, at different concentrations, was tested on the migration of leucocytes by using the sealed capillary migration test. Vasoactive intestinal peptide, at 10(-7)-10(-9)M, inhibited, while at 10(-12)-10(-14)M, stimulated mononuclear leucocyte migration. The migration of polymorphonuclear leucocytes was inhibited by vasoactive intestinal peptide at 10(-6)-10(-9)M, a stimulation was found at 10(-13)-10(-14)M. The inhibiting effect of vasoactive intestinal peptide on leucocyte migration was abolished when vasoactive intestinal peptide was split into C- and N-terminal fragments, while a stimulating effect was retained in the N-terminal fragment, at 10(-14)M, for mononuclear cells. Helodermin and peptide T, as well as two other members of the secretin-glucagon family, secretin and gastric inhibitory peptide, had no effect on the migration. When VIP antiserum was tested, it had an inhibiting effect, which was not seen with control serum, supporting a physiological effect of the lower vasoactive intestinal peptide concentrations. Vasoactive intestinal peptide seems to have dual effects on mononuclear and polymorphonuclear leucocyte migration and, generally, intact vasoactive intestinal peptide seems to be needed for these effects.