Nude mice carrying human squamous‐cell carcinoma xenografts were given i.v. injections of radiolabelled monoclonal antibodies (MAbs). MAb E 48, which reacts with squamous‐cell carcinomas, was labelled with 131l,while a second control MAb of similar immunoglobulin subclass was labelled with 125l. Both antibodies were injected simultaneously, then the mice were scanned with a gamma camera or their tissues were removed and antibody uptake was calculated as a percentage of the injected dose. Uptake of E 48 reached a peak value of 16%/g on day 3, while uptake of the control antibody was less than 1.8%/g. By 24 hr after injection tumor could be visualized without subtraction techniques. At days 3 and 7, only xenografts were visible on imaging. These findings suggest that E 48 is capable of high specificity in targeting isotopes to squamous‐cell carcinomas in an experimental setting.