Radiolabeled GA-17, a murine monoclonal antibody that reacts specifically with glioma cells, bound to a small-cell lung cancer (SCLC) cell line NCI-H69 derived from neural cells, both in vitro and in vivo. The affinity constant of GA-17 F (ab')2 fragment binding to NCI-H69 was 1.02 x 10(8)/M while that to the glioma cell line U87MG was 1.22 x 10(8)/M. Iodine-125-labeled GA-17 F(ab')2 fragments injected i.v. localized well in NCI-H69 cells xenografted in nude mice. The percentage of the injected dose per gram accumulated in the xenografted tumor was 6.87 +/- 1.34% g-1 (mean +/- SD, n = 5) 24 h after injection. On the other hand, control monoclonal F(ab')2 fragments accumulated in the xenografted tumor at 0.75 +/- 0.30% g-1. The tumor-to-blood ratio was 1.8 for NCI-H69, while that of control F(ab')2 was 0.60. In conclusion, the radiolabeled GA-17 F(ab')2 fragment is expected to be useful clinically to visualize the small-cell lung cancer and in radioimmunotherapy.