Nitrocellulose discs, untreated or immersed in paraffin oil or in Vespa orientalis venom, were inserted s.c. in mice. Similarly treated mice were given daily i.v. injections of levan. The discs were extracted, defibrinated, stained and the cells attached were counted. Paraffin oil and venom increased the number of monocytes on the discs. Treatment with levan reduced the number of polymorphs and giant cells, but increased the number of monocytes. Thus levan, which inhibits transendothelial migration of polymorphs, enhances the migration of monocytes.