Monoclonal immunoglobulins (molg) have repeatedly been described in organ and bone marrow transplantation. Although their exact significance is not known, their occurrence is often associated with intensive immunosuppression. We investigated whether molg reflect T-cell immune defect and B-cell activation in renal transplant recipients. Immunofixations and lymphocyte subset analysis (CD4, CD8, CD19) were performed in 182 renal transplant recipients. Soluble CD23 concentrations were measured in patients with molg and in control transplant patients without molg. Monoclonal immunoglobulins were identified in 54 patients (29.6 %). Transplant endurance was shorter (62 +/- 53 months vs 81 +/- 47 months; P < 0.02) and age was older (53 +/- 13 years vs 46 +/- 13 years; P < 0.005) in patients with molg. Maintenance immunosuppression did not differ between patients with and without molg. Mean CD4-cell count was significantly lower in patients with molg (387 +/- 286/mm(3) vs 538 +/- 341/mm(3); P < 0.005). Both CD8- and CD19-cell counts were similar for the 2 groups. Soluble CD23 concentrations were higher in patients with abnormal immunoglobulin values than in patients with normal immunofixation (12.8 +/- 8 vs 1.9 +/- 1.8 microg/l; P < 0. 005). Our study provides new evidence that molg reflect T-cell immune defect in renal transplant recipients. Further studies are required to determine whether CD4-cell count and sCD23 may help to predict the risk of lymphoma in transplant patients with molg.