The protective value of maternal antibodies against exposure by contact to virulent swine fever virus was investigated in a group of 40 piglets at the age of 33-40 days. The piglets were from a single farm and derived from 10 sows which had been vaccinated once with C-strain virus 5.5 months before farrowing. Four contact controls of approximately the same age as the test-groups and born from non-vaccinated sows died 4-10 days (av. 7.5 days) after the onset of symptoms. Three pigs from the group under test succombed to swine fever after an illness of 12 to 16 days (av. 14 days) and one animal died from coli-enteritis. A febrile reaction (av. duration 2.9 days) was observed in more than 75% of the piglets that survived the challenge and all survivors showed a serological response. There was no relationship between the serum-neutralisation titer before challenge and the presence or absence of fever. Attempts to isolate swine fever virus by means of leucoyte cultures, bonemarrow cultures and plasma clot cultures from tonsils, spleen and lymphnodes of surviving piglets 5-7 weeks after the start of the experiment were not successful. It is concluded that a high proportion of the piglets born from dams vaccinated with C-strain virus are protected against contact exposure till the age that they can successfully be vaccinated themselves.