Spheroids of a small-cell lung cancer (SCLC) cell line POC were used to evaluate the uptake and penetration of two antibodies recognising different SCLC antigens. Spheroids approximately 300-400 microns in diameter were incubated with 1 microgram ml-1 125I-labelled NY.3D11, an antibody which reacts with the cluster 1 group antigen (neural cell adhesion molecule; NCAM) and [125I]SWA11, which binds to the cluster w4 antigen. The rate of uptake of both antibodies was similar; an initially rapid phase was seen during the first 8 h and maximum uptake occurred by 24 h. The mean uptake per spheroid at 24 h was 0.97 ng for [125I]NY.3D11 and 0.45 ng for [125I]SWA11. An objective measurement of antibody penetration into spheroids was developed using a computerised image analysis of immunostained sections of spheroids. The concentration of antibody and incubation times were varied. Both antibodies penetrated the spheroids to a depth of 50 microns after 30 min. This increased to about 100 microns after 4 h incubation with 1 or 100 micrograms ml-1 SWA11. The results with 1 microgram ml-1 NY.3D11 were similar, but in the presence of 100 micrograms ml-1 NY.3D11 penetration into the spheroid was deep and diffuse. These results demonstrate a major concentration-dependent difference in the uptake and penetration of cluster 1 and cluster w4 antibodies in this spheroid model and they have implications for the selection of antibodies for targeted therapy of SCLC.