A technique has been described for the demonstration of a human complement component by an immunofluorescent method. The component detected is ß1C-globulin, a moiety of the third complement component, which has previously been obtained in pure form and to which a specific antiserum has been prepared. It has been shown in a model system that the binding of ß1C-globulin as shown by immunofluorescence is strictly equivalent to complement fixation as assessed by standard serological methods. This technique has been applied to the detection of in vivo bound complement in pathological human tissues. It was found that in vivo complement binding occurs in the lesions of several human diseases, but not elsewhere in the same tissues. In a rather limited survey of diseases that has been carried out, in vivo complement binding was found particularly in systemic L.E., various nephritides, and amyloidosis, as well as in single cases of some other diseases. The spectrum of in vivo complement binding has been compared with that of γ-globulin binding (7S and 19S types) and with the demonstration of in vitro complement fixation and rheumatoid factor fixation. It was distinct from each of these. Rheumatoid factor fixation, detected by anti-19S antiserum showed promise as a method for the detection of antigen-antibody complexes and aggregated γ-globulin in tissue sections. The interpretation of these findings in regard to the nature of the binding sites, and their possible significance in regard to pathogenic mechanisms have been discussed.