Recombinant human IL-13 is chemotactic for purified human peripheral blood monocytes Cell migration is stimulated with a typical bell-shaped concentration curve and is maximal at 10 ng/ml. Migration is the result of chemotaxis, and not chemokinesis, as shown by checkerboard experiments. The chemotactic activity of IL-13 on monocytes is not inhibited by preincubation of the cells with pertussis toxin but is diminished by preincubation with protein kinase inhibitors. The related cytokine, IL-4, also stimulates migration of monocytes in the Boyden chamber assays at concentrations similar to those effective for IL-13. Human IL-13 is capable of attracting rabbit peripheral blood monocytes at those concentrations active on human monocytes. On the other hand, no neutrophil migration was induced by IL-13, even at 1 microM concentrations.