The small DNA tumour virus simian virus 40 (SV40) expresses only two proteins in the cells that it has transformed, large-T and small-t. Large-T has an apparent molecular mass of 94 000 and genetic evidence indicates that it is necessary both for the induction and maintenance of the transformed state. In the lytic cycle of the virus the large-T protein acts to initiate viral DNA replication and appears to regulate its own transcription. These properties imply that the protein must interact with existing host cell mechanisms in a specific way and that an understanding of these interactions may be important in discovering how the protein acts in `transforming' non-permissive cells. To identify these interaction structures in the host cell an immunochemical approach has been adopted, and has shown that a fraction of the large-T protein in transformed cells exists in a specific complex with a host-coded phosphoprotein. The host protein has an apparent molecular mass of 53 000. The physiological role of this host protein-T antigen complex has been examined by studying the nature of the complex in a wide variety of different SV40 transformed cell lines. Although in SV40-transformed cells of rodent origin all the host protein present in the cell is complexed to large-T, it was found that in human cells transformed by SV40 only a fraction of the host protein is present in a complex and these cells contain large-T and the host protein in both the free and the complexed state. In SV40-transformed mouse cells that have been selected for loss of the transformed phenotype the complex was still present and in apparently normal amounts. This would indicate that no simple causal relation exists between the presence of the complex and the phenotype of the cell containing it.