Eotaxin is a novel C-C chemokine purified from the bronchoalveolar lavage (BAL) fluid of actively sensitised guinea-pigs after aerosol allergen challenge. In this study we show that eotaxin induced a dose-dependent increase in both intracellular-free calcium concentration ([Ca²⁺]_i) and aggregation of guinea-pig eosinophils in vitro. Intradermally injected eotaxin induced the accumulation of [¹¹¹In]eosinophils in naive guinea-pig skin in vivo, without oedema-inducing activity; the latter emerging in separate BAL fluid HPLC fractions. Aerosol exposure of naive guinea-pigs to eotaxin in vivo caused a selective increase in eosinophils in BAL fluid. Thus, eotaxin activates guinea-pig eosinophils in vitro and causes a selective eosinophil accumulation in the lung in vivo. Eotaxin and related molecules are potentially important endogenous signalling substances in allergic reactions in vivo.