A consistent modification in B lymphocyte activation has been observed 1 month after infection of C3H/HeJ mice with scrapie. The mitogenic response to lipopolysaccharide of splenocyte cultures from experimental mice was reduced 30 to 60% as compared to controls. This reduction in mitogen responsiveness was transient but coincided with the onset of detectable splenomegaly and with the reported recovery of maximum yield of infectious scrapie agent in the spleen. The DNA synthetic response to lipopolysaccharide stimulation of splenocytes from scrapie-infected C3H/HeJ mice was depressed relative to controls only between 20 and 40 days after intracerebral inoculation. At all other times, experimental and control responses were identical. Scrapieassociated decreases in mitogenesis were found whether the spleen cell cultures contained splenocytes from individual mice, splenocytes pooled from several mice, or gradient-purified mononuclear cells. The responses of C3H/HeJ splenocyte cultures to phytohemagglutinin or concanavalin A stimulation were unaffected by scrapie infection.