Methods are described by which immunochemical procedures such as immunodiffusion and immunoelectrophoresis can be used to provide quantitative measurement of the concentration of individual proteins in fluids such as serum, spinal fluid, tissue extracts and the like. That is accomplished typically by making quantitative optical measurements of the zones of precipitation which are produced, in parallel with those of standard preparations containing known concentrations of the proteins to be determined; deriving from the measurements of the precipitation zones selected parameters; and comparing the selected values of the experimental and reference preparations. The zone measurements are preferably made by scanning the plates electronically, with suitable arithmetic manipulation of the resulting video values at a plurality of selected positions for each zone.