Eosinophil stimulation promoter (ESP) is a soluble product of activated mouse lymphoid cells which increases in vitro migration of mouse eosinophils. Pretreatment of the lymphoid cells, either by freezing and thawing or the lytic action of antithymocyte serum, abolished the ability of these sensitized cells to produce ESP when challenged by specific antigen. Its production is inhibited by the addition of puromycin. ESP is relatively heat stable, with full activity remaining after incubation at 60°C for 30 min, but none after 100°C for 30 min. Dilution markedly reduces its activity. ESP is sensitive to a proteolytic enzyme, chymotrypsin, but insensitive to RNase and to Vibrio cholera neuraminidase. Molecular sieve column chromatography of ESP-containing culture supernatant fluids yields a single peak of ESP activity, associated with only a small fraction of the total protein. The estimated molecular weight of ESP is 24,000 to 56,000 daltons.