In this study, results are presented which are in agreement with predictions made on basis of the 'three-stage hypothesis' on the development of benign monoclonal gammapathy (BMG). In a T-cell depletion model. C57BL/Ka nude mice were shown to develop single and multiple homogeneous immunoglobulins (H-Ig) during aging in the highest frequencies known so far. Ninety per cent of the C57BL/Ka nude mice displayed one or more H-Ig at 12 months of age. In a T-cell supplementation model, infusion of corticosteriod resistant T cells into 9-month-old BALB/c nude mice resulted in a decrease in the frequency of H-Ig from 43% at 9 months down to 20% at 15 months of age. In contrast, the frequency of H-Ig in the control group increased from 40% at 9 months up to 68% at 12 months. The results show that normally functioning T cells are essential for the generation of a normal, heterogeneous Ig spectrum; they further support the validity of the three-stage hypothesis with regard to the role of an impairment of the T immune system in the pathogenesis of BMG.