Subadult ICR mice were infected with the low virulent Langat virus TP21 E5 strain clone "14" belonging to the tick-borne encephalitis (TBE) complex by subcutaneous (s.c.) and intracerebral (i.c.) routes. From 5 to 6 days post infection (p.i.), no virus was detected in cultured brain fragments of mice, which received 10(6) ic LD50 into interscapular area. Acute lethal encephalitis with lesions confined to the vicinity of the inoculation area (parietal cortex, basal ganglia, thalamus) has developed in all mice, which received greater than or equal to 3 PFU of the virus by i.c. route. However, no virus was recovered from the cultured fragments of brain stem and cerebellum of these animals, although direct isolation attempts were regularly positive from brain cortex and basal ganglia. Survivors, which did not succumb to i.c. administration of approximately equal to 1 ic LD50 (0.3 PFU) of the attenuated Langat strain were autopsied between 53-74 days p.i. Attempts to isolate the virus from cultured fragments of brain cortex and basal ganglia remained negative despite of the presence of focal residual histological lesions in g. hippocampi in 15% of of animals examined.