The effect of lymphocytes from patients with newly diagnosed insulin dependent diabetes on isolated rat or human islets during a 20 h in vitro incubation was investigated by morphological and biochemical methods. All stages of target cell reaction of lymphocytes against beta-cells were observed. Cytoplasmic projections towards and contacts with beta-cells, circumscribed lysis of the outer cell membrane at the contact side and complete lysis of beta-cells were seen. Such findings could not be registered with alpha- or other non beta-cells and in control experiments using lymphocytes from healthy persons. In some cases the lymphocytes were phenotyped using monoclonal antibodies by the indirect immuno-gold technique. It could be demonstrated that lymphocytes in contact with necrotic beta-cells were of the CD8+ve subset. In addition the morphological investigations were supplemented by quantitative biochemical studies measuring the non-secretory insulin release ("cytotoxic" release) from islets into the culture medium and their ability to respond to a consecutive stimulation by arginin with insulin and glucagon secretion. The mean cytotoxic insulin release was 11.3 +/- 1.5 ng/islet/20 h (n = 25) in the diabetic group versus 0.56 +/- 0.15 ng/islet/20 h (n = 15) in the control subjects (alpha less than 0.001). Functional testing of the islets following incubation with lymphocytes from diabetic patients showed diminished insulin (58.6 +/- 5.1%, n = 18, alpha less than 0.001) and unchanged glucagon response (103.0 +/- 3.4%, n = 10, n.s.) when compared with untreated islets (= 100%).(ABSTRACT TRUNCATED AT 250 WORDS)