A rapid method for the direct conjugation of affinity-purified antibodies with fluorescein (termed DCAPA) is described. This procedure involves the immobilization of antibodies as antigen-antibody complexes on nitrocellulose blots, and subsequently the bound antibodies are reacted with fluorescein isothiocyanate. An enriched sample of smooth muscle tropomysin transferred to nitrocellulose paper by the Western blotting procedure has been used as the affinity medium for purification of specific tropomyosin antibody from whole rabbit antiserum. Direct conjugation of the antibody with fluorescein was carried out following the binding of antibody to antigen. Direct conjugation and affinity purification of antibodies directed against tropomyosin was accomplished in 2-3 d using an enriched tropomyosin sample and whole antiserum directed against tropomyosin. The immunofluorescence images obtained with this procedure exhibit distinct advantages with regard to background fluorescence and overall specificity of antibody binding. The usefulness of this direct conjugation method in various experimental protocols is discussed.