Two mouse monoclonal antibodies (MoAbs), TP-1 and TP-3, previously shown in immunohistochemical studies to react with osteosarcomas, were labelled with 125I or 131I and evaluated for their ability to localise to human osteogenic sarcoma xenografts after intravenous injection. The radiolabelled TP-1 and TP-3 MoAbs had immunoreactive fractions of 70% and 67%, respectively, and bound to target cells with binding constants of 8.5 X 10(8) M-1 and 4.0 X 10(9) M-1, respectively. After injection of labelled TP-3 IgG, approximately 16% of the dose X g-1 tissue was found in the tumour after 24 hours. Maximum tumour/blood radioactivity ratios of 6-7 were achieved 3-4 days after antibody injection, while the ratios for the normal tissues were less than 1. The tumours could be clearly visualised by whole-body gamma scintigraphy without the need for subtraction techniques. The TP-1 IgG accumulated to a large extent also in the spleen. Hence, with this antibody the tumour was less well delineated from the adjacent normal tissues. However, the F(ab')2 fragments, derived from the TP-1 IgG, gave tumour/blood ratios up to approximately 40 after 3-4 days and yielded sharp gamma scintigrams of the tumour. Specificity of the antibody localisation was indicated by the lack of accumulation in a contralateral melanoma xenograft and the failure of 2 isotype-matched irrelevant MoAbs to localise to the sarcomas. With the F(ab')2 fragments satisfactory images could be obtained already after 16 hours. The results suggest that this preparation may be useful in clinical radioimmunodetection of osteogenic sarcomas.