A soluble cadmium-binding protein, with properties similar to metallothionein, has been isolated from rabbit alveolar macrophages. The macrophages were cultured in Medium 199 with Earle's salts for 24 hr in the presence of 10 micromoles CdCl2 and carrier-free 109Cd as a tracer. The isolation procedure began with application of a 100,000 g cell supernatant to a column of Sephadex G-75 Fine. The fraction containing the greatest amount of cadmium was eluted at a relative elution volume, Ve/Vo, of 1.87. A molecular weight determination performed following Sephadex chromatography indicated that the apparent molecular weight of the impure protein was approximately 11,000. The fractions containing cadmium were pooled and purification procedures were applied, including acetone fractionation, DEAE-cellulose chromatography, and polyacrylamide gel electrophoresis. DEAE-Cellulose chromatography following acetone fractionation indicated the presence of two forms of metalloprotein as has been demonstrated previously in the isolation of cadmium-thionein from liver and kidney. The two forms of metalloprotein were subjected to polyacrylamide gel electrophoresis and, although separation was incomplete, bands obtained corresponded to those typically observed in rat liver.