The monoclonal antibodies 2H4 and 4B4 are specifically directed against CD45RA and CD29 antigens, respectively, which are expressed on both CD4+ and CD8+ lymphocytes. We used them to monitor the distribution of these T-cell subsets in 19 recent-onset IDDM patients (26 +/- 7 yr of age [mean +/- SD throughout]) receiving cyclosporin, and who had been treated with insulin for < 1 mo. Blood samples were examined before starting cyclosporin and after 3 and 9-12 mo of therapy. CD45RA and CD29 are expressed on distinct subsets of CD4+ T-cells, whereas in both healthy subjects and IDDM patients, 2H4 and 4B4 labeling does not always differentiate two distinct CD8+ populations. Various proportions of double-labeled CD8+ CD45RA+ CD29+ cells were detected in 59% of IDDM patients and 70% of healthy control subjects. Before cyclosporin treatment, recent-onset IDDM patients had a significantly lower proportion of CD4+CD29+ cells than the healthy control subjects (42 +/- 11 vs. 52 +/- 9%, P < 0.004). This imbalance corresponded to a significant increase in the CD4+ CD45RA+/CD4+ CD29+ ratio in the IDDM patients (1.37 +/- 0.93 vs. 0.78 +/- 0.36, P < 0.014). Independent of the presence of double-labeled cells, the proportion of CD8+ lymphocytes expressing CD45RA was significantly higher in the patients than in control subjects (72 +/- 15 vs. 57 +/- 15%, P < 0.004). The mean density of the CD45RA antigen (but not that of CD29) on both CD4+ and CD8+ cells was significantly higher in the IDDM patients before treatment than in the healthy control subjects. Cyclosporin induced complete or partial remission of the disease in 12 of 19 patients at 4-6 mo of treatment, but did not correct the imbalance between the regulatory T-cell subsets, regardless of clinical effectiveness. Cyclosporin use was associated with a significant decrease in CD45RA antigen density on both CD4+ and CD8+ T-cells.