This article describes the steps taken to verify that the tick Ixodes ricinus is 
the main vector of Slovenian meningo-encephalitis.In 1954, several unsuccessful 
attempts were made to isolate the virus from unengorged ticks collected at 
random, but in the following year this system was abandoned, and instead the 
search for specimens was carried out on the basis of evidence supplied by 
meningo-encephalitis patients with a tick-bite case history.In June-the peak 
endemic period-140 unengorged ticks were collected from a part of a forest in 
central Slovenia where one of these patients had been working. The ticks were 
washed in alcohol, finely ground, and centrifuged, and the supernatant was 
inoculated intracerebrally into suckling mice, and intraperitoneally into adult 
mice and guinea-pigs. The infected suckling mice died between the fourteenth and 
eighteenth day after inoculation; a suspension of their brains was inoculated 
intracerebrally into a further batch of suckling and adult mice, which 
subsequently fell ill, showing signs in the central nervous system.The virus 
strain obtained was subjected to neutralization and complement-fixation tests, 
in order to identify the infective agent.