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Suppression of plaque former cell (PFC) response to sheep red blood cells (SRBC) following Rauscher leukemia virus (RLV) or Friend leukemia virus (FLV) infection has been well documented in several mouse strains sensitive to these viruses (1-3). Likewise, a transient suppression of PFC response has been reported for the virus-resistant C57B1/6 mouse during the first week after FLV infection (4). Ceglowski and Friedman (5) compared PFC immunosuppression by FLV in several strains 8 days after infection and reported that the degree of immunosuppression roughly paralleled that of virus susceptibility, with the only hybrid tested (the B6D2F 1 of the C57B 1/6 and the DBA) showing even more immunosuppression than its sensitive DBA parent. However, in that study as in most of the previous studies the prime concern was with the immune response of strains known to be sensitive to the virus. Further, many of these studies have been limited in their observation periods to the rather short duration after infection in which confirmation of immunosuppression is usually obtained. On the other hand, recent data of at least one study (4) suggest that stimulation of PFC response by leukemia virus might also be demonstrable in resistant strains if the observation period is sufficiently long. In the present study we have compared the level of PFC response as affected by RLV in the sensitive SJL/J mouse, the resistant C57B 1/10 mouse and their resistant hybrid-(SJL/JxC57B l/lO)Fl. Evidence for stimulation of PFC response in the resistant mice between the second and fourth week after ~~

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Exposure to the chemical carcinogen, methyl methane sulfonate, enhanced leukemogenesis in mice given threshold doses of Friend leukemia virus, as shown by peripheral white blood cell counts, splenomegaly and survival.

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