To study the relationships between the responses to viral antigens and the HLA-DR3 and -DR4 associations in Type 1 (insulin-dependent) diabetes mellitus, the frequency of T-lymphocyte proliferating in response to mumps, Coxsackie B4 and varicella-zoster antigens was determined. A decreased frequency was found in T lymphocytes able to respond to mumps or Coxsackie B4 when presented together with DR3, as compared with the frequency of T lymphocytes able to respond to these viruses together with other DR determinants. This was not found for varicella-zoster or purified protein derivative of tuberculin. In contrast, an increased frequency was found in T lymphocytes responding to mumps or Coxsackie B4 together with DR4, compared with other DR determinants. The results were similar in Type 1 diabetic and healthy individuals. The results suggest that elements on the DR3 and DR4 molecules may control T-lymphocyte responses to mumps and Coxsackie B4 viruses.