A resonance light scattering (RLS) imaging technique was introduced to measure the light scattering of aggregation species induced by proteins, and thus a method of detecting proteins in the range of picograms was proposed. In acidic medium, J-aggregation of alpha,beta,gamma,delta-tetrakis(p-sulfophenyl)porphyrin (TPPS(4)) in the presence of proteins occurs, resulting in strong RLS signals characterized at 490 nm. Under the excitation of a 488-nm light beam of argon ion laser source, the scattered light of single J-aggregation species could be observed with a common microscope, and the images could be captured with a cooled charge-coupled device camera. Data analysis for the digital images showed that the counts of aggregate species in the detection focus plane are proportional to the concentration of proteins in picograms. When 1.0 x 10(-7)M TPPS(4) was employed, 0.01-210 ng/ml bovine serum albumin and human serum albumin could be detected with limits of detection lower than 10 pg/ml (3 sigma). Three human blood serum samples were satisfactorily detected with relative standard deviations lower than 3.04%.