A number of studies of inflammation and of cell growth and transformation have recently converged by defining two related families of cytokines. The first, represented by macrophage inflammatory protein 1, is composed of several gene products that have been identified in activated T cells, macrophages, and fibroblasts. The biological activities of this family are still being characterized but so far include effects on neutrophils, monocytes, and hematopoietic cells. The second, represented by macrophage inflammatory protein 2, includes platelet products such as platelet factor 4 and beta-thromboglobulin as well as several other recently described gene products that have effects on a number of cell types including neutrophils, fibroblasts, hematopoietic cells, and melanoma cells. The two families are structurally related and may have evolved from a common ancestral gene that duplicated and then diverged. Their differential control and expression in a wide variety of cell types suggests that they may have multiple functions in regulating inflammation and cell growth.