Mouse 6D6 IgG2a and 5B5 IgM monoclonal antibodies which specifically bind murine lung carcinoma cells (3LL cells) were injected to healthy and tumor-bearing mice. In vivo localization was analyzed by counting the tissue radioactivity and by external gamma ray scintigraphy at various times after IV injection of 125I- or 131I-labeled antibodies. The clearance of the two monoclonal antibodies was not modified by the presence of the tumor, and the 6D6 IgG2a was cleared at a rate slower than the 5B5 IgM. Both antibodies gave a high specific uptake at the tumor level; the tumor-to-healthy tissue ratios were higher with the 6D6 IgG2a than the 5B5 IgM; unspecific mouse immunoglobulins (IgG2) did not localize in the tumor. The amount of 6D6 IgG2a antibody still associated with the tumor after 2 days following IV injection was 10 times higher than that of 5B5 IgM, and was still high enough to localize the tumor after 5 days. Imaging experiments confirmed the ability of 6D6 IgG2a to detect the presence of tumor cells. The targeting kinetics determined by computer analysis of camera images indicated a rapid targeting of antibodies in tumor with a maximal concentration after 4-6 h; after 48 h the background was quite low and the 6D6 IgG2a appeared to be specifically localized in the tumor.