The effect of circulating antigen (human kappa light chains (LC)) on radioimmunoscintigraphy was examined in a normal rat model. Rats were implanted subcutaneously with kappa (test) and lambda (control) LC coupled to sepharose beads (500 ��g LC/ml swollen gel). When 131I-labelled monoclonal antibody, K-1-21 was injected, clear images of specific antibody localisation to the kappa implant were seen 6 days later. The kappa implant: blood ratio (K:B) was 7.69��0.45, which represented an implant uptake of 2.52��0.20 percent injected dose per gram (%ID/g). When 131I-K-1-21 was preincubated with 125I-kappa LC at ratios of up to 1:10 antibody: antigen, (w/w) before injection; or if antigen was given daily by bolus injection or continuously infused via an osmotic pump, very clear specific images of antibody localisation to the kappa implant were still obtained. There was, however, a decline in the absolute uptake of label by the kappa implant. The greatest change was observed following continuous antigen infusion, when kappa implant uptake fell to 1.26��0.1 %ID/g. Nevertheless, the K:B ratio was still maintained at 6.8��0.2, due to increased clearance of antibody from the blood. Thus, in this case, where antigen and antibody might be expected to form small, soluble immune complexes, there was no major deleterious effect on radioimmunoscintigraphy. �� 1989 Springer-Verlag.