I have reviewed the current status of microinjection based on fusion of red blood cells and tissue culture cells. Macromolecules are introduced into red blood cells during hypotonic hemolysis, and the resealed red cells are then fused to tissue culture cells with Sendai virus. The procedure has been used to inject ferritin, thymidine kinase, bovine serum albumin, and transfer RNA molecules into large numbers of tissue culture cells. Physiologically significant amounts of various macromolecules can be transferred, and preliminary studies show that [125I]bovine serum albumin and transfer RNA are stable within recipient culture cells. Tissue culture cells remain viable following microinjection. Red cell-mediated microinjection should facilitate the study of various processes, such as macromolecular turnover and genetic regulation, that are not easily studied with conventional biochemical techniques.