A rapid thin-layer immunoaffinity chromatographic method for quantitation in serum of an acute phase reactant, C-reactive protein (CRP), which can differentiate between viral and bacteria] infections, is described, where material and reagent costs are minimal. The analysis is based on the "sandwich" assay format using monoclonal antibodies directed against two sites of CRP. One of the antibodies is covalently bound to defined zones on a thin-layer immunoaffinity chromatography membrane, while the other antibody is covalently bound to deeply dyed blue latex particles. After incubation (CRP sample and latex particles), the CRP-latex immunocomplex is allowed to migrate along the immunoaffinity chromatography membrane. In the presence of antigen, a sandwich is formed between the CRP-latex immunocomplex and membrane-bound antibodies, which results in the appearance of blue lines on the membrane. Antibody immobilization on the TLC membrane is made with a redesigned piezoelectric-driven ink-jet printer. The time required for the analysis is less than 10 min. Quantitation is achieved either by counting the lines visually, with scanning reflectometry, or with a modified bar code reader. The limit of detection was estimated in the low femtomolar range using the naked eye as detector.