The MT-4 human T-cell line expresses HTLV-1 Tax and is permissive for replication of an HIV-1 gp41 mutant lacking the cytoplasmic tail. MT-4 cells (lot 150048), distributed by the NIH AIDS Reagent Program (NIH-ARP), were found to be Tax-deficient and unable to host replication of the gp41-truncated HIV-1 mutant. These findings, together with short tandem repeat (STR) profiling, established that lot 150048 are not MT-4 cells.