A method of quantitative analysis of microscopic biological specimens in a fluid medium is disclosed in which the specimens are rendered magnetically responsive by immunospecific binding with ferromagnetic colloid. A known quantity of magnetically-responsive marker particles are added to the fluid medium. The fluid medium is then subjected to a magnetic separation process, to collect the magnetic species from the fluid. The collected species are resuspended in a second fluid medium, and the relative quantities thereof are enumerated to determine the concentration of the desired biological specimen in the first fluid medium. The marker particles may comprise magnetic particles having a relatively large magnetic moment, a magnetic moment approximately equal to the magnetically-labelled biological speciment of interest, or both in order to compensate the determination for variations in immunospecific binding affinity and/or magnetic collection efficiency. The present method is useful for conducting hematological assays, such as red blood cell counts and related determinations. In the case of red blood cells, the specimens may be rendered magnetically responsive by reduction of the iron atoms present in hemoglobin. The enumeration step of the method may be conducted with such equipment as a flow cytometer, or by a novel optical scanning magnetic cell immobilization apparatus.