A commercially available alpha-particle spectrometer and 210Po alpha-particle source were used to determine the mass of microgram quantities of biomolecules. Samples were deposited in microliter volumes on thin silicon nitride windows and dried. The energy loss of the alpha-particles after traversing the sample was converted to a mass using tabulated alpha-particle stopping powers. The measurement was absolute, independent of biomolecule species, and no standards were needed for quantitation. The method has a dynamic range of 0.1-100 microg for deposits of diameter 1-2 mm. The precision varies from approximately 20% at 100 ng to a few percent at 5-100 microg. The silicon nitride windows allow multimodal analysis of the same quantified sample, including PIXE probing of elemental abundances, molecular identification by mass spectrometry, and isotopic quantitation of interactions. The method was used with accelerator mass spectrometry to quantify specific activities of microgram quantities of 14C-labeled proteins.