Susceptibilities of a continuous rhesus monkey kidney cell line (MA-104) and that of a human colon carcinoma cell line (HT-29) to infection by different human and animal rotavirus strains were compared. HT-29 cells appeared to be more sensitive to human rotavirus infection than MA-104 cells, whereas the latter cell line was more susceptible to animal rotavirus replication. The greater sensitivity to human rotavirus infection of HT-29 cells was confirmed by the successful, direct isolation of these viruses from faecal specimens. Human rotavirus infection of HT-29 cells was also followed by transmission electron microscopy. In ultra-thin sections, unenveloped particles of rotaviruses, representing infectious mature virions, were observed in large number. Moreover, many "double-shelled" particles were detected in negative-stained supernatants from infected cultures. Scanning electron microscopy of uninfected HT-29 cells showed that in the presence of Ca++, required for rotavirus growth, they are able to express some of the features of mature intestinal cells. In view of these results, HT-29 cells appear to be a useful in vitro model for the study of rotavirus infection.