Hamsters injected subcutaneously with a single "low-dose" inoculum (10 mouse LD50) of Powassan virus developed viremia titering 106.2 mouse LD50 per milliliter of blood whereas hamsters receiving a "high-dose" inoculum of 105.0 mouse LD50 of virus developed a viremia of only 104.5 mouse LD50 per milliliter.Hemagglutination-inhibiting (HI) antibodies were first detected 7 days following the "low-dose" inoculation and attained maximum titers of 160. The HI antibody response to the "high-dose" virus inoculation began on the 5th day, at a time when viremia was present, and antibody titers did not exceed 40. Sucrose-density analysis and 2-mercaptoethanol treatment of sera revealed that IgM antibodies were induced by both inocula but animals receiving a low dose of virus produced higher IgG responses than did animals receiving a high dose of virus. Neutralizing antibodies, which did not appear until 30 days post inoculation, were present at a log neutralizing index (NI) of 2.0 at 60 days in animals receiving 10 mouse LD50 of virus and at a log NI of 1.0 in those receiving 105.0 mouse LD50. The NI of IgG was equal to the index for total gamma globulin in samples that contained both IgM and IgG as demonstrated by the HI test.Animals originally exposed to a "low-dose" inoculum produced a more prolonged anamnestic response when challenged 63 days later with 105.0 mouse LD50 of virus than did animals receiving a second inoculum containing only 103.0 or 10 mouse LD50 of virus. In contrast, poor secondary antibody responses were elicited by challenge doses of 105.0, 103.0, or 10 mouse LD50 of virus in animals that originally had received a "high-dose" of virus. IgM and IgG classes of antibody were induced in all secondary responses and the log NI of each group of animals was elevated by about 1 during anamnesis.