Antibodies titres against Coxsackie-B (CB) and mumps virus (MV) were determined in 180 normal subjects and two groups of diabetics, 236 with diabetes of recent onset (DRO) and 108 with diabetes of long duration (DLD). Positive antibody titres to these viruses were found more frequently in controls than in diabetics. In fact, the percentage of positive titres was significantly less than in controls in several instances particularly in juvenile onset diabetes of long duration. These data suggest that CB and MV are not causally related to human diabetes mellitus.