Nude rats heterotransplanted with human melanoma metastasis were injected subcutaneously on the hind paw with 125I-labelled monoclonal antibody 96.5 and with control antibody 131I-OKT3. The elimination from the injection site followed a biexponential function. The uptake in the inguinal lymph nodes on the side of the injection was initially high, but after 90 h it equalled the control side. The uptake in the tumour was slower than after i.v. injection but higher than in other tissues except blood. More than 80% of the activity in the dissected liver represented circulating blood. The uptake ratio of 96.5/OKT3 was c.3 in the tumours but c. 1 in all other tissues including blood. The capillary filtration coefficient was proportional to the uptake in organs like liver, lungs and muscle. It is concluded that subcutaneously injected radiolabelled monoclonal antibodies are initially transported via the lymph but then mainly distributed via the blood reaching the different tissues including tumours.