The oncogenes coding for the Harvey murine sarcoma virus p21ras protein as well as those coding for myc, myb, and mht products were fused to the amino-terminal portion of the bacteriophage lambda cII gene on the expression vector pJL6. In addition two regions of the gene for the human T-cell leukemia virus subgroup I (HTLV-I) envelope were expressed in our bacterial system. Each of 11 human sera tested that had been shown to contain antibodies to HTLV-I or -II by an enzyme-linked immunosorbent assay recognized the bacterially synthesized envelope proteins. No reaction was detected when 17 control sera were tested. This system will be useful for large-scale seroepidemiological surveys for HTLV-I and related human retroviruses. The other oncogene products expressed in our bacterial vector system also demonstrated specific immunoreactivities. In addition to this feature the bacterial ras protein was seen to bind guanosine diphosphate and was capable of autophosphorylation. Taken together these data suggest that the proteins produced with high efficiency by the bacterial expression system can be immunologically recognized as antigens and can in part perform some of their associated biochemical functions.