Use the word count CMD under utilities limited studies in acutely infected people suggest that a non-syncitiuminducing, macrophage-tropic variant of HIV is transmitted by sexual contact. The study described here was undertaken to determine if rhesus macaques inoculated by different routes are infected with distinct viral variants. Five mature female rhesus macaques were intravaginally (IVAG) inoculated with 103 TCID50 of SIVMAC251 and 5 male rhesus macaques were intravenously (IV) inoculated with 103 TCID50 of SIVMAC251. The blood samples from the inoculated animals were separated into plasma and PBMC. The CD14+ PBMC, cCD4- PBMC and unfractionated PBMC from the inoculated animals were cocultivated on cemx174 cells, rhesus macaque macrophages and T cells. The target cell populations were obtained from the blood of 3 normal rhesus macaques whose cells were permissive for SIV replication. At 3, 7 and 14 days pi, macrophage-tropic SIV was isolated from the plasma, PBMC, CD14- and CD14+ cells of all the animals. At 7 days pi, T cell-tropic virus was isolated from the plasma, PBMC or CD14- PBMC of 3 IV inoculated animals and one IVAG inoculated animal. At 14 days pi, T cell-tropic virus was isolated from the plasma and PBMC of 4 IV inoculated animals and one IVAG inoculated animal. These results demonstrate that the virus stock used for the inoculations contains both T-cell and macrophage tropic SIV variants. Both T cell tropic and macrophage tropic strains could be isolated from the blood of the IV inoculated animals but not from IVAG inoculated animals. The inability to detect a T cell tropic virus in acutely infected intravaginally inoculated animals provides the first direct evidence that macrophage tropic SIV variants are selected by mucosal transmission.