Recent advances in biotechnology have allowed the production of new types of macromolecular therapeutic agents (antibodies, immunotoxins, cytokines, extracellular matrix molecule (ECM) proteins, vectors) that may eventually find broad clinical applications in the treatment of human tumors and other diseases. The model of the Multicellular Tumor Spheroids (MTS) represents a valuable tool to test the therapeutic potential of these new pharmacologic agents in a 3-D context. Specific questions pertaining to the behaviour in a 3-D setting of some of the macromolecules under evaluation for in vivo applications can also be addressed in the MTS model (e.g. 'binding site barrier', role of cell-cell and cell-ECM interactions). This paper reviews the most significant contributions regarding the delivery of macromolecules to MTS, the penetration and therapeutic effects of antibodies, radiolabelled antibodies, immunotoxins and other macromolecular compounds.