RECOGNITION of antigen in the specific immune response is well understood. Recognition of foreign or altered self material by the cells which mediate non-specific immunity has received little attention. In tissue injury or bacterial infections, many proteins may become enzymatically degraded or conformationally altered. Here I report a study of the chemotactic migration of neutrophil leukocytes towards the model proteins, haemoglobin and myoglobin, in their native and haem-free conformationally altered forms, and show that conformational changes in these proteins are recognized by leukocytes and evoke a chemotactic response, although the native proteins are inactive. A model for the molecular basis of this recognition is proposed.