Embryonic stem (ES) cells have the ability to self-renew, execute multiple lineage paths, and dominantly reprogram differentiated cells upon cell fusion. Here, we describe an approach that reprograms human B lymphocytes toward pluripotency by generating inter-species heterokaryons with mouse ES cells. This induces a human ES-specific gene expression profile, in which the extent and the rapidity of conversion allows us to compare the capacity of different mouse ES cell lines to dominantly induce pluripotency. This approach, coupled with pharmacological inhibition, gene knock-out, or knock-down permits factors that are required to directly induce reprogramming to be defined individually, as well as in combination. Experimental heterokaryons provide a simple and tractable approach to address the mechanisms underlying direct reprogramming to pluripotency. The procedure requires 5 days to complete.