Monensin in ruminant diets increases production of propionic acid. We have tested the hypothesis that propionic acid may be elevated to such an extent by monensin that it cannot be optimally metabolized by the methyl malonyl-CoA pathway requiring vitamin B12 (B12) in the liver. Thus, the effects of weekly B12 injections (10 mg X head-1 X wk-1, intramuscularly) with and without dietary monensin (25 mg/kg diet) on average daily gain (ADG), dry matter intake (DMI), feed to gain ration (F/G), liver and serum B12 concentrations and liver activity of propionate metabolizing enzymes were examined in an 84-d trial. Sixteen lambs (27.5 kg average initial wt) were assigned randomly to one of four treatments in a factorial arrangement: monensin plus B12, monensin without B12, no monensin plus B12 and no monensin without B12. Lambs were fed an 80% concentrate diet and slaughtered at the end of the trial. Liver samples were obtained by biopsy on d 0 and at slaughter on d 84 to determine activity of propionate metabolizing enzymes and B12 concentrations. Serum samples were taken on d 0, 28, 56 and 84 to determine serum B12 concentration. Neither monensin nor B12 affected (P greater than .10) ADG, DMI or F/G. Lambs receiving B12 had higher (P less than .01) serum B12 concentrations, but this was not reflected (P greater than .10) in higher liver B12 concentrations. No difference (P greater than .10) in liver propionate metabolizing activity among treatments was detected; however, monensin decreased (P less than .05) fumarate and malate formation by liver homogenates. Liver B12 concentrations were highly correlated with endogenous propionate metabolizing activity at d 0 (r = .73, P less than .01) and d 84 (r = .51, P less than .05). Results suggest no advantage to providing supplemental B12 to lambs fed monensin-supplemented, high-concentrate diets.