The present invention relates to the transient modification of cells. In particular embodiments, the cells are immune systems, such as PBMC, PBL, T (CD3+ and/or CD8+) and Natural Killer (NK) cells. The modified cells provide a population of cells that express a genetically engineered chimeric receptor which can be administered to a patient therapeutically. The present invention further relates to methods that deliver mRNA coding for the chimeric receptor to unstimulated resting PBMC, PBL, T (CD3+ and/or CD8+) and NK cells and which delivers the mRNA efficiently to the transfected cells and promotes significant target cell killing.
