D3 Serum by Premier Research Labs is a rich supply of live-source natural vitamin D3. This formula may help to support the cardiovascular and immune systems. The serum also may help with calcium absorption for strong bones and teeth. As a live-source D3 vitamin‚ this formula is absorbed by the body quickly. It is believed vitamin D3 plays a crucial role in supporting many of the body's systems. That includes your bones, brain and heart as well as breast tissues, the prostate and your immune system.