To evaluate the effect of IGF‐1 on the autoimmune process of d cell destruction, permissive non‐obese diabetic (NOD) recipients were adoptively transferred with 7 × 10⁶ autoreactive T cells from diabetic NOD mice and were administered subcutaneously 10μ;g rhIGF‐1, twice daily for 3 weeks. Administration of rhIGF‐1 reduced the final incidence of successful transfers of diabetes observed in only 6/24 mice (25%) versus 12/21 (57%) in control mice. A marked reduction of insulitis during histological analysis of pancreatic glands was also observed. Mice treated with rhIGF‐1 had a higher percentage of intact islets (486 ± 12% versus 16 ± 1·1%. P= 0·001) and a lower percentage of infiltrated islets. Islets from rhIGF‐1‐treated mice had a more intense insulin staining reflecting a higher β cell mass, but no difference was observed in the amount of insulin content of pancreatic extracts and in the amounts of mRNA transcripts for proinsulin. No difference was also observed in the titres of three islet cell antibody (ICA)‐positive sera and in the pattern of A2B5 staining. Some mice developed diabetes and severe islet cell infiltration despite rhIGF‐1, thus indicating that some committed T cells were still able to invade the islets and cause β cell destruction. The percentages of CD4⁺ and CD8⁺ T cells in the spleen of experimental mice were similar. To evaluate the effects of rhIGF‐1 on cell trafficking in recipient mice, T cells from diabetic NOD Thy‐1,2 mice injected into congenic NOD‐N Thy‐1,1 mice were monitored 3 weeks after adoptive cell transfer. The percentage of Thy‐1,2⁺ T cells was significantly reduced in the spleen (10·8 ± 1·3%versus 17·2 ± 3·9%, P= 0·004) of rhIGF‐1 treated mice in contrast to the thymus (68·4 ± 7·9%versus 72·87 ± 6·2%, P= 0·306), suggesting that rhIGF‐1 could influence T cell trafficking to the lymphoid organs. The findings that rhIGF‐1 has protective effects in autoimmune diabetes opens new perspectives for future experiments as well as for preventive strategies in human type I diabetes.