Proteoglycan extracted from the Kurloff cells of the guinea-pig spleen and its corresponding glycosaminoglycan inhibits the migration of macrophages from capillary tubes in vitro, but the proteoglycan has no effect on lymphocyte transformation in vitro. A proteoglycan, chemically and spectrally similar to Kurloff cell proteoglycan, extracted from human spleen, also inhibits macrophage migration. In contrast, proteoglycan from cartilage and its corresponding glycosaminoglycan had no significant effect. The differences between Kurloff cell proteoglycan and migration inhibitory factor (MIF) are discussed.