The sulfonated shale oil extract, Ichthyol, was studied for its effect on the migration of human neutrophilic granulocytes by the Boyden chamber technique. When presented to the cells in a concentration gradient, Ichthyol induced a directed migration. There was little or no chemokinetic effect of Ichthyol when added to the cell compartment of the Boyden chamber. The chemotactic migration towards the tripeptides, formyl-methionyl-leucylphenylalanine and formyl-norleucyl-leucyl-phenylalanine, towards the arachidonic acid-derived eosinophil chemotactic factor released from neutrophils by the ionophore A 23187, and towards complementderived chemotactic activity of normal human serum was inhibited or abrogated by Ichthyol. The Ichthyol effect on f-MLP chemotaxis could be partly overcome by excessive f-MLP concentrations. It was reversible when Ichthyol-incubated cells were washed and resuspended in regular buffer. It is suggested that various substances contained in Ichthyol interacted with either the chemotactic factors or the cell membrane or both and thus blocked cell stimulation. The results could help to explain the cell accumulation and abscess formation observed with Ichthyol in inflammatory skin lesions and the anti-inflammatory properties of the drug.