Eighteen cotton mice (Peromyscus gossypinus) were divided into four groups: seropositive and seronegative adults, and subadults with and without a history of maternal antibody. After intraperitoneal inoculation of 0.5 ml of Fe3-7C virus suspension containing 800 MLD50, virus was recovered from the blood of 2 of 4 nonimmun adults and from both groups of subadults. Viremias persisted up to 4 days in some individuals with mean titers ranging from 7.4 log10 MLD50 per ml on day 1 to 3.9 log10 MLD50 per ml on day 4. Virus was not recovered from the group of seropositive adults, 2 of 4 seronegative adults, or the uninoculated seronegative contact animals of each group. Hemagglutination-inhibiting antibodies were detected in all mice 8 days post-inoculation, with titers ranging from 1:20 to 1:320 by day 35.