A panel of heterohybridomas secreting human IgG and IgM monoclonal antibodies to the C, c, G, E, and e antigens of the Rh system has been established. Both classes of antibody have been shown to react with red blood cells carrying their respective antigens; those of the IgM class being able to directly agglutinate unmodified red blood cells in saline. The heterohybridomas have been shown to be suitable for bulk culture and produce levels of antibody in the range of 15-73 micrograms/ml in the spent culture supernatant.