This paper describes mathematical models and computer programs developed for quantitative microprobe analysis of thin biological materials and red blood cells, and presents the results of their application to sets of red cells from Apollo 16 astronauts and from a potassium-rubidium ion exchange experiment. Derivations of bulk and thin film models are based on the classical state probability density function for penetrating electrons and on electron transport characteristics at the specimen substrate interface. Red cell analysis is accomplished using the thin film model and a current averaged red cell mass thickness obtained from statistical estimates of the probe current and red cell mass distributions. Computer analysis of the experimental data yields red cell set averages and distributions which follow the temporal changes in red cell potassium content and agree substantially with tabular and flame photometer results.