Duc-Nguyen, Huu (The Children's Hospital of Philadelphia, Philadelphia, Pa.), and Werner Henle. Replication of mumps virus in human leukocyte cultures. J. Bacteriol. 92:258-265. 1966.-Human peripheral leukocyte cultures maintained in the presence of phytohemagglutinin (PHA) were found to support to some extent the replication of mumps virus. When such cultures were exposed, within 24 hr after their initiation, to a high input multiplicity of virus, successful infection, as determined by immunofluorescence and plaque assays, did not become evident before the 3rd or 4th day. On exposure of cultures 4 to 5 days old, viral replication was detectable within 2 days. In both instances, peak immunofluorescence and virus titers were reached when the cultures were 7 to 9 days old and composed mainly of blast forms. With decreasing input multiplicities of infection, cells containing viral antigen and production of infectious viral progeny became detectable with increasing delay. No significant viral replication was noted in surviving cells maintained in the absence of PHA. These results indicate that mainly, if not solely, the PHA-stimulated cells of the lymphocytic series support viral multiplication. The extent of the infectious process was limited, however, because the life span of the cultures was not significantly shortened, the yields of infectious virus per immunofluorescent cell were at all times low, and most infected cells contained only a few well-delineated small masses of antigen, suggestive of an abortive infection. Only fresh cultures were capable of synthesizing interferon on stimulation by mumps, Newcastle disease, or Sendai viruses. When the cultures were set up in the presence of PHA, this capacity was lost within 24 hr. PHA per sefailed to induce detectable production of an interferon under the conditions used. The implications of these findings are discussed.