Metal-binding chelators may interact with biological systems by either of two mechanisms: they may combine with an essential metal, which can be either freely dissociated or part of an enzyme prosthetic group, or they may react with a metal ion to form a biologically reactive metal-chelate complex. As trace metals are always present as contaminants in serum-supplemented culture media used to study chelating agents, it is frequently difficult to distinguish between the two possibilities. Here we describe the use of a nontoxic, copper-specific chelating agent, bathocuproine sulphonate (Fig. 1) which, by combining with available endogenous copper in a tissue culture preparation, abolished the toxicity of three structurally unrelated chelating agents. These three agents may therefore be considered to be biologically active by the second mechanism.

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