Material for the study ofvariation in the Malayan Langat virus (TP-21 strain).

The lack of genetic uniformity of the Malayan Langat virus, TP-21 strain isolated from Ixodes granulatus ticks in Malaya, was demonstrated.Increased pathogenicity of the virus for white mice and rhesus monkeys after passage through chick-embryo fibroblast cultures at temperatures of 36 degrees C and 40 degrees C is due to the selection of the pathogenic virus particles which are always present in the initial brain-tissue suspension of the TP-21 Langat strain.The frequency of the occurrence of pathogenic clones and their selection rate was studied in relation to the conditions of cultivation. A study was also made of the genetic stability and basic biological properties of pathogenic and non-pathogenic clones of the Langat virus.