In view of a more and more rational approach to cancer treatment, knowledge of the interaction between the various therapeutic modalities is indispensable. The present article tries to examine whether there are special immunological aspects in tumour cell populations which are resistant to chemotherapy. For example, taking into account some well established concepts, it is generally felt that the possibilities of the biological agents should be primarily exploited against limited number of neoplastic cells, when other weapons, including chemotherapy, have already produced a major debulking of the tumour masses. In this view, the residual disease, which has often become chemoresistant, should be endowed with an adequate sensitivity to the biological approach. Encouragingly, a series of experimental data indicates that, when compared with their sensitive counterparts, the chemoresistant cells show equal or even increased sensitivity to various components of the immunological equipment. However, other than in this direction, perhaps there are other promising opportunities for the biological agents and they are discussed in this article; among these, there is the possibility to confer or restore drug sensitivity by immunological means.