OBJECTIVE Concentrated aqueous solutions of hydroxocobalamin (OHCob) are given intravenously for the treatment of cyanide poisoning. Because OHCob solutions are intensely red and have peak light absorptions at 352 nm and 525 nm, we investigated whether the presence of OHCob in serum would interfere with various automated, colorimetric chemistry measurements. METHODS Selected serum chemistry colorimetric measurements were compared in seven patients, using their own serum as control, with serum containing OHCob at the following concentrations: 100 mg/L, 500 mg/L, and 1,000 mg/L. These concentrations are in the range achieved with therapeutic doses of OHCob when given for cyanide poisoning. RESULTS Statistically significant alterations in serum values for aspartate aminotransferase, total bilirubin, creatinine, magnesium, and iron were seen in the presence of OHCob. CONCLUSIONS The presence of OHCob in serum interferes with several chemistry methodologies, and such interference should be anticipated when this antidote is used.