Peripheral blood cell counts of mice during the preleukemic phase of Friend virus infection. The (A) leukocytes (WBC), (B) hematocrit, and (C) platelets were measured at various times after infection on day 0. The mice had been fed either folate-free diet or control diet at 2 weeks before infection. The folate-free diet was switched to control diet at 2 weeks after infection in the folate-deficient 1 group and at 6 weeks after infection in the folate-deficient 2 groups. The mice in each group that survived for 52 weeks without signs of leukemia were killed at that time. Data are ± 1 standard error of the mean (SEM) for 10 to 50 mice.