The purpose of this study, using multi-cell spheroids as an in vitro model of micrometastases of ovarian carcinoma for i.p. radio-immunotherapy, was to measure the uptake and retention kinetics of 111In-labeled F(ab')2 fragments of OC125 monoclonal antibody (MAb) in spheroids of the NIH:OVCAR-3 cell line and to estimate absorbed doses with beta-emitting radionuclides (kinetics was assumed to be similar to that of indium-111). With 0.2-mm-diameter spheroids at different MAb concentrations the highest binding value was determined. Retention kinetics showed a biological half-life of 50 hr. These data were used to calculate absorbed doses by integration of Berger's latest-point kernels. Mean absorbed doses when spatial distribution was considered to be uniform at the surface (no penetration) or throughout the spheroid (total penetration) were, respectively, 247 and 417 Gy with 153Sm, and 90 and 135 Gy with 90Y. Thus, the use of a similar MAb concentration and specific activity in patients should lead to high absorbed doses in i.p. radio-immunotherapy of micrometastases.