A scanning transmission electron microscope is employed to determine the mass of biological macromolecules. Elastically scattered electrons are collected by an annular detector that is capable of counting single electrons. Off-line processing of these dark field micrographs stored on a magnetic tape is accomplished by a mini-computer. It allows the number of electrons scattered by spherical or filamentous proteins to be evaluated. The calibration factor relating the number of scattered electrons to the mass of the protein is derived from scattering theory and is experimentally determined from biological macromolecules of known mass. Mass-loss kinetics of biological specimens due to the electron beam are measured for various protein structures. The application of this method is illustrated by determination of the mass of an oligometric protein (major phage T4 head protein) and the mass per unit length of a filamentous protein aggregate (F-pili). The unique possibilities of this new technique as well as its limitations are discussed.