Two HIV-1 isolates were obtained from a patient receiving long-term treatment with zidovudine (ZDV). The in vitro sensitivity to ZDV triphos-phate of the reverse transcriptase (RT) from both isolates appeared to be unchanged compared to that of the LAV-Bru HIV-1 reference strain. When isolates were grown in CEM cells (a T-lymphoblastoid tumor cell line) and their RT activity and core antigen (p24) production were determined, the level of p24 production compared to RT activity was high; in infected CEM cells treated with ZDV, RT activity was at background level while the p24 production was still significant, thus indicating a dissociation of RT activity and core antigen production.