This invention relates to processes which are used to produce, isolate, and characterize human rotavirus/animal rotavirus reassortants and to produce live attenuated vaccines and vaccine precursors. In the present strategy there is involved the new use of either (1) high titer hyperimmune antisera or (2) monoclonal antisera to select reassortants with the desired human phenotype. A point of novelty is the finding that antiserum or monoclonal antisera alone, so long as it possesses high titer neutralizing activity against only the 34-38Kd glycoprotein or of the animal parent, is sufficient to use for selection of reassortant rotaviruses with human phenotype. Also, the novel products are live attenuated vaccine precursors and vaccines.