Based on the finding reported in the preceding paper (Kanbayashi, et al.: J. Biochem) that subunits of islets-activating protein (IAP), a new protein purified from the culture media of Bordetella pertussis, were inactive as such, but regained the original biological activities when recombined, the conditions required for recovery of the biological activities were studied. Essentially the same biological activities as the native IAP were recovered when the smallest subunit, F-3, was incubated with one of the other subunits, F-1 and F-2, at a pH of around 7, at temperatures below 30 degrees C and for longer than 12 h. During the incubation, association products were formed which were isolated by gel filtration as homogenous proteins that consisted of two subunits probably in a molar ratio of 1 : 1. The native IAP (consisting of two IAP subunits including F-3) were equipotent in enhancing insulin secretory responses, in inhibiting epinephrine-induced hyperglycemia, in inducing leukocytosis and in increasing histamine sensitivity in experimental animals.