The ganglioside GD2 is highly expressed on human tumors of neuroectodermal origin. We investigated by scanning electron microscopy the ganglioside GD2 distribution on the surface of spheroids of the neuroblastoma cell line SK-N-LO. About 50% of cells showed GD2 on the plasma membrane and the distribution of GD2 on most of these cells was heterogeneous, with more GD2 at the contact sites of the cells. The binding kinetics of the chimeric anti-GD2 antibody ch14.18 labelled with I-125 on spheroids (average diameter: 450 microns) was determined by gamma counting. Over 4 hours the antibody concentration was raised substantially but up to 24 hours there was only a very slow further increase. A clustered pattern of bound chimeric anti-GD2 antibody ch14.18 was found on a cross-section of the spheroid by autoradiography.