Azathioprine‐induced lymphocytic neoplasms of NZB mice lack ecotropic murine leukaemia virus

NZB mice injected intramuscularly throughout a 6‐month period with the immunosuppressant azathioprine (Imuran) developed lymphocytic lymphomas 6–7 months after treatment was initiated. These malignancies were quite distinct from the reticulum‐cell neoplasia which occurs spontaneously in the strain, and were readily transplantable to NZB or histocompatible BALB/c recipients. Xenotropic, but not ecotropic murine leukaemia virus (MuLV) was detected in leukaemic tissues of some donor and recipient NZBs when tested in vitro by co‐cultivation with permissive cell lines, genome rescue, XC and viral polymerase assays. Virus filtrates prepared from donor leukaemic tissues were non‐pathogenic when injected into newborn C3H mice. These results are evidence against a mandatory ecotropic MuLV genome in lymphocytic neoplasia.