Using a simple fluorescence test, we show that commercially prepared colloidal gold complexes with goat second antibodies often contain free active antibody. Because such antibodies will compete with antibody-colloidal gold particles for antigen binding sites, labeling intensity at the ultrastructural level must necessarily be submaximal to an unknown degree with such preparations. A survey of five preparations suggests that the problem may be widespread. We recommend that a test of the sort described be incorporated routinely into protocols with all colloidal gold products.
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Kramarcy et al. (1991) studied this question.