Natural killer (NK) cell lines are difficult to transfect using standard techniques, which limits the ability to establish long-term knockdown of proteins with short-hairpin (sh)RNAs. We have developed a method to stably knockdown protein expression in human NK-like lines by introducing shRNAs in retroviral vectors. After a single transduction with retrovirus, shRNA-containing cells can be selected with drug treatment or sorted for enhanced green fluorescent protein (EGFP) expression. With this method, protein expression can be stably decreased to less than 10% of wild-type levels.