The cytoreductive effects of anti-transferrin receptor (anti-TfnR) immunotoxins (ITs) and of ricin toxin against tumour micromasses have been evaluated in a multicellular tumour spheroid (MTS) model. More than 600 (656) MTSs obtained with human breast carcinoma (MCF7) or rat glioblastoma (9L) cell lines were treated individually with ITs or toxin and the effects induced by the treatment were measured for each MTS as volume variation vs time by applying the Gompertz growth model. Two dose-dependent patterns of MTS growth were observed in MTSs of both cell lines in response to IT or toxin treatment: (1) complete inhibition of MTS growth ('sterilisation'); and (2) partial/complete inhibition ('heterogeneous response'). Within the range of IT or toxin concentrations resulting in partial inhibition of MTS growth, the sensitivity of treated MTSs was extremely heterogeneous (the cytoreductive effects varying between 0.1 and 4 logs of cells killed for a given IT or toxin concentration). Analysis of the post-treatment regrowth kinetics indicated that treated non-sterilised and control MTSs reached the same final limiting volumes. However, the doubling time estimated for the surviving cells of treated MCF7 and 9L MTSs ranged between 15 and 50 h, indicating that each MTS had individual growing potential. In conclusion, our results indicate that at substerilising IT concentrations individual heterogenicity of MTSs may greatly influence the cytoreductive potential of ITs. An implication of our study is that the efficacy of an IT treatment in eradicating disseminated micrometastases may not be predictable a priori. The MTS model that we describe in this paper may help in dissecting out factors limiting the effect of ITs in three-dimensional tumours.