To investigate a putative immunoregulatory defect in T1D patients, we assessed the frequency of two B cell subsets with suspected regulatory function, CD19+CD27−CD24hiCD38hi transitional B cells and CD19+CD5+ (CD5+ B cells), as well as CD19+CD27+IgD+IgM+ (marginal zone) B cells. Here, CD19 is linked to type 1 diabetes mellitus.