The development of the lesions caused by Argas persicus larvae while feeding on chickens, their natural hosts, was followed using histological techniques. Penetration of the skin by the mouthparts was accompanied by rapid lysis of the cells of the epidermis and dermis within 100 mum of the mouthparts. A foreign-body reaction developed within 2 mm of the attachment and was characterized by rapid dilation of blood vessels, emigration of leucocytes, particularly heterophil leucocytes (heterophils) into the tissues and oedema. Haemorrhage developed near to the mouthparts and within 2 h of the start of feeding, erythrocytes were seen in the gut caeca of the larvae. Whole blood provides the diet of the larva throughout the entire period of feeding and unlike the situation described with nymphal and adult. A. persicus the blood is lysed immediately after ingestion. By 30 h the extra-vascular heterophils had migrated to the mouthparts around which they formed a loose, superficial, collar. Further emigration of heterophils into the tissues then ceased, although some were present in the blood being ingested. It appears that the heterophils mask the mouthparts so that they are no longer recognised as foreign by the host and thus provide an example of the concept of adaptation tolerance. But the development of lymphocyte and monocyte infiltrations in the dermis close to the mouthparts indicates that the immunological response of the host to salivary secretions is not suppressed. Oedema which develops around the site of attachment further protects the larvae from the host's predations.