Propagation of the Naples and Sicilian types of Phlebotomus (sandfly) fever viruses in primary human and mouse kidney tissue cultures has been described. Plaques in human kidney cultures were delayed but were produced consistently by both Naples and Sicilian types. Cytopathogenic effects as a result of multiplication by either virus type, could be demonstrated in infant kidney cultures only after an adaptation period. In contrast, cytopathic changes could be seen in the first and subsequent virus passages when mouse kidney cultures were inoculated with either the Naples or Sicilian virus types. Mouse kidney cultures were found to be more susceptible to virus infection than human kidney monolayers.