Viral progeny of the molecular clone 19k1 of feline immunodeficiency virus (FIV) can infect feline T-cells but not Crandell feline kidney (CrFK) cells. In contrast, the biological isolate FIV-AM6c, which was CrFK adapted by co-cultivation of FIV-AM6 infected thymocytes with CrFK cells, can infect both thymocytes and CrFK cells. The envelope gene of FIV-AM6c was amplified by polymerase chain reaction using DNA from infected CrFK cells, and subsequently cloned and sequenced. To map viral determinants of CrFK cell tropism, chimeric viruses with a 19k1 background containing envelope gene fragments of FIV-AM6c were constructed. CrFK cells were transfected with DNA of these chimeric clones and co-cultivated with thymocytes. After 3 days the CrFK cells and the thymocytes were cultured separately. FIV antigen could be detected in most of the thymocyte cultures within 14 days and in one of the CrFK cultures after 52 days. The resulting virus from this CrFK culture can infect both CrFK cells and thymocytes. The results of this study indicate that the envelope region contains determinants of CrFK tropism. The delay in replication indicates that also determinants other than those identified here are involved in CrFK cell tropism. More chimeric clones are being studied at present to map these determinants.