The purpose of the article is to give the experience gained with the manufacture of the vaccine for five years and to dicuss the potency results obtained. Sixty ampoules of WI38 cells were used. The processing of twelve of them was stopped for accidental reasons. Forty-eight lots were freeze-dried, four were rejected as final product, three lots because of low potency, one following some febrile reactions in children due to an unexpectedly high content in endotoxin. The activity level of each lot was determined: (1) by titration of the viral suspension before concentration in young mice by the intracerebral route or by the plaquing technique in monolayers of BHK 21 cells. The mean titer in mice is 10(6.5) per ml; on cells the titer is lower, mean 10(5.8) per ml, but more homogeneous, and (2) by the NIH potency test in mice: 90% of the lots have an antigenic value compared to the International Reference Vaccine above 2. There is not a good agreement between the results of the NIH potency test and the titer of the viral suspension. The antigenic values obtained on the freeze-dried vaccine after storage one month at 37 degrees C are the same as those for the vaccine stored at + 4 degrees C. The activity is not modified after storage for two years at + 4 degrees C in the freeze-dried or in the liquid state. The main drawback of the vaccine is its high cost. Some suggestions are proposed to try to lower it and to find a sound compromise between high quality and price.