A strictly controlled epidemiological trial was applied to the study of the incidence of neurological complications in children inoculated with vaccine from the L-IVP strain of the 2nd, 3rd, and 5th passages (generations). To ascertain a possible connection between the neuropathogenicity with the infectious activity the persons vaccinated were divided into 3 groups in accordance with the infectiousness level of the preparation obtained: the I group--3--5-10(8)OOU/ml; the II group--6--8-9-10(8) OOU/ml; the III group over 9-10(8) OOU/ml. The least number of neurological complications was recorded among children primarily inoculated with the vaccine of the 3rd passage with the infectiousness titre of over 9-10(8) OOU/ml.