Gibbon ape leukemia viruses (GALV) are a group of retroviruses which have been associated with hematopoietic neoplasms in primates. Two of the viruses, GALV-SEATO and GALV-San Francisco (GALV-SF), are associated with myeloid and lymphocytic leukemias, respectively, in apes. Using an assay based on the transient expression of the bacterial gene chloramphenicol acetyltransferase (CAT), we examined the transcriptional activity of GALV-SEATO and GALV-SF. The results suggest that high level expression of GALV is due primarily to cis-acting enhancer sequences. Sequence delineation analysis of GALV-SEATO showed the GALV-SEATO enhancer sequences to be located within a 45-bp tandem repeat in GALV-SEATO. GALV-SF, which has two- to fivefold lower transcriptional activity, contains only a single copy of the 45-bp element with 6-bp differences from those in the GALV-SEATO enhancer element. This 45-bp element is highly homologous to sequences within the LTRs of several murine leukemia viruses but has not been examined for enhancer function in these retroviruses. Expression of GALV was not restricted to hematopoietic cells but was extraordinarily high in MLA 144 cells, a gibbon ape T-cell line known to be infected with GALV-SF. However, expression of constructs containing the CAT gene directed by GALV-SEATO LTR sequences was similar in uninfected and GALV-infected fibroblasts, indicating the lack of virally encoded or virally induced trans-activating factors capable of increasing expression in these cells.