About fifty diarrheic fecal samples in which the calf rotavirus was present in variable quantities were passed in tissue cultures and the ability of the virus to replicate was analysed under different experimental conditions by means of an indirect immunoflourescent test. Several cell species were shown to be susceptible to the virus but the best results were obtained in primary or secondary cultures of embryonic calf kidney cells in presence of maintainance medium containing 2 to 4 p. 100 foetal calf serum or chick serum. The specific infectivity of this type of virus is low in tissue culture. Under the best experimental conditions about 70 p. 100 of the samples that were positive for the virus by electron microscopy, induced the appearance of specifically fluorescent cells. Cytopathic effects due to infection are extremely discreet and most often there is no evidence of transfer of infection from cell to cell. Different attempts to adapt the infective agent to grow in tissue culture finally succeeded in the selection of a virus that was able to induce fluorescnet plaques at the fifth pass.