The first morphological and biochemical findings on the effect of leukocytes from a newly diagnosed type I diabetic patient on isolated rat islets are presented. Leukocytes from venous blood were co-incubated with isolated neonatal rat islets for 22 h. According to the biochemical and electron-microscopical findings, beta cells localized in the periphery of the islets and a few single beta cells show lytic alterations (single cell necroses). The electron micrographs suggest that the beta cells were lysed after contact with lymphocytes or lymphoid cells from the diabetic patient (target cell reaction). The simultaneous biochemical and morphological investigations reveal that after beta cell lysis, insulin released in a granular state can be phagocytosed by granulocytes and thus escape the estimation of insulin concentration in the medium.