Thirteen patients with metastatic breast carcinoma were given injection of 50-1593 .mu.g of 131I-monoclonal antibody (MAb) B6.2 immunoglobulin G and F(ab'')2 for pharmacokinetic evaluation and radioimmunoimaging. Blood clearance of the 131I-MAb-B6.2 was biphasic. The mean half-times (t1/2.alpha., t1/2.beta.) for the immunoglobulin G were 3.5 .+-. 1.7 and 20.9 .+-. 11.0 h, respectively. The t1/2.alpha. for the F(ab'')2 was 1.7 .+-. 1.3 h, and the t1/2.beta. was 31.0 .+-. 5.7 h. The percentage of protein bound 131I for the immunoglobulin G and for the F(ab'')2 at 72 h was 73.7 .+-. 11.4% and 58.2 .+-. 14.5%, respectively. In vitro reactivity of MAb B6.2 with granulocytes isolated from normal subjects and patients was demonstrated by cytofluorometric and radioimmunoassays. MAb B6.2 was shown to bind with normal cross-reacting antigen, a cell surface antigen known to be expressed on normal human granulocytes, Reactivity with normal cross-reacting antigen on granulocytes is consistent with the skeletal images obtained during immunoscintigraphy of all 13 patients. A specific tumor image was observed in one patient. No toxicity was encountered. In spite of extensive preclinical data suggesting that 131I-MAb B6.2 would be a useful agent for radioimmunoimaging, the clinical utility of this reagent is probably limited because of the reactivity with granulocytes.