Several previous studies have demonstrated a chemotactic factor for white blood cells in extracts from psoriatic scales. In the present work, the question was studied whether chemotaxis is a primary or a secondary event in the development of a psoriatic lesion. The migration-stimulating properties of polymorphonuclear leukocytes of different extracts were determined by means of a modified Boyden technique. Aqueous extracts of psoriatic scales were good stimulators of migration of both autologous and homologous leukocytes. Uninvolved skin adjacent to psoriatic plaques (perilesional skin) has previously been shown to differ from control skin in various respects. Extracts of such perilesional skin of psoriatic patients did not have migration-stimulating properties. This result indicates that chemotaxis is a secondary event in the pathogenesis of psoriasis.