Aqueous solutions of stroma-free human haemoglobin are being evaluated as potential oxygen-carrying resuscitation fluids. There are indications, however, that such solutions may produce toxic side-effects in vivo. Stroma-free haemoglobin solution produced a 50% fall in mean arterial pressure when infused into a small animal model despite containing very low levels of non-haem protein and phospholipid contaminants. This effect was not produced by haemoglobin solutions after extensive dialysis. Red cell-derived adenine nucleotides were found to be present in concentrations high enough to cause such a response (80-85 micrograms/ml). We have developed a chromatographic assay capable of predicting hypotension in our animal model and consider that the complete absence of adenine nucleotides must be confirmed in all studies concerning the possible toxic side-effects of stroma-free haemoglobin solutions.