Abstract
Pig kidney cells of the PS or PK2a line have proved useful for the study of group B arboviruses (Westaway, 1966; de Madrid & Porterfield, 1969). While following the course of arbovirus replication in PS cells by thin section electron microscopy, it became clear that the PS cell line was carrying a non-cytopathic virus morphologically indistinguishable from the oncogenic C-type virus particles of murine leukaemias (Bernhard, 1960; Dalton, 1962). The PS cell line is known to be chronically infected with swine fever virus (Shimizu et al. 1969), and to avoid this possible complication, attention was next turned to another pig kidney cell line, PK 15, which is known to be free from swine-fever infection. Numerous C-type particles were found in thin sections prepared from control PK 15 cells. To exclude the possibility that both PS and PK 15 cells had become infected, possibly with a murine virus, during handling in this Institute, a fresh supply of PK 15 cells was obtained from the American Type Culture Collection (designation CCL 33, passage 133).