Microcolonies were obtained by culturing cells of two human osteosarcoma lines (OHS and KPDX) and one human melanoma line (WIX-c) for either 24 or 72 h. The microcolonies were treated with either alpha-particle radiation emitted by the 211At-labelled monoclonal antibody (MAb) TP-3 or external beam X-rays. Survival of microcolonies was assayed by colony formation. Therapeutic gain factor (TGF) values were calculated for two survival levels, 50% and 20% microcolony regeneration (i.e. at least one cell in 50% or 20% of the colonies survived the treatments). The TGF values were affected by the specific activity of the 211At-MAb conjugate, the antigen expression of the cells and the size and growth pattern of the microcolonies. Treatment with 211At-TP-3 gave TGF values that varied from 1.3 +/- 0.4 to 4.5 +/- 0.7 (mean +/- s.e.). The antigen-rich OHS cell line had on average 1.6 times higher TGF than the antigen-poor KPDX cell line. The TGF increased significantly with colony size for the densely packed colonies of the KPDX cell line but not for the OHS cell line, which had colonies with cells growing in a more scattered pattern. Control experiments with the two non-specific 211At forms, free 211At and 211At-labelled bovine serum albumin, gave TGF values from 0.6 +/- 0.1 to 1.0 +/- 0.3. This study suggests that in vivo evaluation of 211At-MAbs using relevant tumour models is desirable.