An affinity purification technique has been developed using mild elution conditions for the isolation of humanized murine and murine (\mathrm{IgG}{1}). This technique is based on the innate affinity of (\mathrm{IgG}{1}) for metal and utilizes immobilized metal-affinity chromatography. Antibody bound to the metal-affinity column is eluted with a descending (\mathrm{pH}) gradient and (\mathrm{IgG}{1}) elutes around (\mathrm{pH}) 6.0. Humanized murine (\operatorname{IgG}{1}) isolated from cell culture media using this procedure is approximately (90 %) pure and does not contain any free light chain, light-chain dimer, or contaminating serum albumin. In addition, murine (\mathrm{IgG}{1}) has been isolated from ascites fluid by direct application to the metal-affinity column. Murine (\mathbf{I g G}{1}) is recovered essentially free from albumin in approximately (60 %) purity, the principal contaminant being transferrin. This method facilitates further purification which is easily achieved by cation-exchange chromatography. The near complete removal of albumin makes metal-affinity chromatography an alternative to salt precipitation of antibody from ascites fluid. Recoveries of antibody from the column are high, typically greater than (90 %). We have identified the location of the metal affinity in the (\mathrm{IgG}_{1}) as being near the carboxy terminus of the heavy chain. A histidine-rich sequence is present in this region which offers the possibility of genetically engineering this sequence to form an even higher affinity metal binding site for potential application in antibody imaging and therapeutics. Ci 1994 Academic Press, Inc.