Type I diabetes [insulin-dependent diabetes mellitus (IDDM)] is an autoimmune disease associated with the destruction of pancreatic beta cells. Serum from patients with IDDM increased L-type calcium channel activity of insulin-producing cells and of GH3 cells derived from a pituitary tumor. The subsequent increase in the concentration of free cytoplasmic Ca2+ ([Ca2+]i) was associated with DNA fragmentation typical of programmed cell death or apoptosis. These effects of the serum were prevented by adding a blocker of voltage-activated L-type Ca2+ channels. When the serum was depleted of immunoglobulin M (IgM), it no longer affected [Ca2+]i. An IgM-mediated increase in Ca2+ influx may thus be part of the autoimmune reaction associated with IDDM and contribute to the destruction of beta cells in vivo.