A procedure is described by which proteins can be rapidly and efficiently microinjected into large numbers of culture cells. Proteins were first introduced into mammalian red blood cells during hypotonic hemolysis, and the resealed red cells were subsequently fused to culture cells using Sendai virus. In seven experiments, thymidine kinase or 125I-BSA were transferred to culture cells during fusion. Although proteins were used in the present experiments, the microinjection procedure should work equally well for other macromolecules.