Alpha-1-microglobulin (A1M) is a low molecular weight heme-binding, anti-oxidant protein that is readily filtered by the glomerulus and reabsorbed by proximal tubules. Given these properties, recombinant A1M (rA1M) has been proposed as a renal antioxidant and therapeutic agent. However, little direct evidence to support this hypothesis exists. Hence, we have sought 'proof of concept' in this regard. Cultured proximal tubule (HK-2) cells, or isolated mouse proximal tubule segments, were challenged with a variety of pro-oxidant insults: i) hemin; ii) myoglobin; iii) 'catalytic' iron; iv) H₂O₂/Fenton reagents; v) Ca²⁺ ionophore, vi) antimycin A; or vii) hypoxia (with/without rA1M treatment). HK-2 injury was gauged by %LDH release and MTT uptake. In vivo protection was sought in rA1M-treated mice subjected to: i) graded myohemoglobinura (2, 4, 8, or 9 ml/Kg glycerol injection); ii) purified myoglobinemia/uria; or iii) endotoxemia. In vivo injury was assessed by BUN, creatinine, and the expression of redox sensitive genes (HO-1, NGAL, MCP-1 mRNAs). Although rA1M totally blocked in vitro hemin toxicity, equimolar albumin (another heme binder), or 10% serum, induced equal protection. rA1M failed to mitigate any non-hemin forms of either in vitro or in vivo injury. A1M appeared to be rapidly degraded within proximal tubules (Western blotting). Surprisingly, rAIM exerted select injury-promoting effects (increased in vitro catalytic iron / antimycin toxicities; increased in vivo MCP-1/NGAL mRNA expression following glycerol or endotoxin injection). We conclude that rA1M has questionable utility as a renal antioxidant/cytoprotective agent, particularly in the presence of larger amounts of competitive free heme (e.g. albumin) binders.