Outbred albino mice were infected subcutaneously with 10(6) amastigotes of Leishmania mexicana mexicana and the subsequent lesions were evaluated by light and electron microscopy at various intervals after infection. The animals developed persistent nodules and a spectrum of lesions of variable size which was correlated with the host's ability to control the parasite in the tissue. During the acute phase of the disease the histopathological results showed an accumulation of granulocytes, some mononuclear phagocytes and a predominance of eosinophils as compared to other cell types. In this early acute phase, eosinophils were found in the tissue together with normal and degranulating mast cells. In the granulomatous inflammatory reaction of the chronic phases, there was infiltration of granulocytes parallel to parasite multiplication and the formation of parasitized vacuolated macrophages. The number of eosinophils was consistently greater than neutrophils, regardless of lesion type or number of parasites present in the tissue. During the acute reaction, the granulocytes apparently destroyed many parasites; however, there was an unvaryingly low level of phagocytosis of amastigotes during the chronic stages by both eosinophils and neutrophils. Neutrophils seemed to be more effective than eosinophils in the killing of ingested parasites. A close association between eosinophils and parasitized macrophages was seen in the chronic lesions; thus, eosinophils might contribute to parasite destruction through co-operation with macrophages.