Five tumours, which arose in cats naturally or experimentally infected with feline immunodeficiency virus (FIV), were examined with molecular probes to establish tumour cell lineage and to screen for integrated viral sequences. Three of the tumours were classed as B-cell lymphomas on the basis of morphology, immunocytochemistry, rearrangement of immunoglobulin heavy chain genes and lack of rearrangement of T-cell receptor (TCR) beta-chain genes. Two of these B-cell tumours arose in specific pathogen-free (SPF) cats experimentally infected with FIV. One case of multi-centric lymphosarcoma came from a cat naturally infected with both FIV and feline leukaemia virus (FeLV). This tumour contained integrated FeLV proviral sequences and was judged to be of T-cell origin on the basis of TCR gene rearrangement. The fifth case was a mast cell tumour. Rearrangement of the c-myc locus was not found in any of the FIV-associated tumours but was shown to be present in a rare immunoblastic B-cell lymphoma which arose in an uninfected SPF cat. None of the FIV-associated tumours showed evidence of integrated FIV sequences by Southern blot hybridisation, despite isolation of infectious virus from in vitro cultures of tumour cells in I case. These results confirm that FIV-associated tumours can occur in the absence of FeLV and suggest that the role of FIV in lymphomagenesis is generally indirect.