Frozen tissue sections of lymph nodes and spleens from twelve patients with nodular (follicular) lymphomas were studied with fluorescein-labeled antisera against human immunoglobulins. Immunoglobulins detected by this method in the neoplastic nodules were minimal or absent. Furthermore, in no instance could the characteristic network pattern of immunoglobulin distribution which is readily demonstrable in active germinal centers be observed within these nodules. It is suggested that the preservation of the follicular network of immunoglobulins as observed by fluorescent antihuman IgG, IgM, or polyvalent antisera indicates functionally intact germinal centers. The detection, by this rather simple method, of large amounts of immunoglobulin distributed in such a distinctive pattern may thus be very useful for the differentiation between severe benign follicular hyperplasia and malignant lymphoma with nodular pattern, which may be difficult to distinguish by routine histological methods.