Research on cysts of the potato-root eelworm is handicapped by the fact that the cyst contents vary considerably in quantity. So it is impossible to characterize the infectivity of a soil sample by determining only the number of cysts present. Though not all larvae are hatched from the cysts in one season, a better impression of the infectivity is got when we know how many living eggs and larvae are present in the cysts.

Oostenbrink (1950) estimates the number of living eggs and larvae by crushing each cyst separately under the microscope. Especially when a large number of cysts is present, this technique takes much time. During our research this estimation appeared to be very subjective; it was almost impossible to get a clear impression of the viability of the cysts, especially when many mouldy or empty eggs were found.

Fenwick (1952) developed the bleaching powder technique in which the presoaked cysts are dissected by means of a Hagedorn-needle (Reid, 1952) and for 80 minutes are exposed to a nominal 1 per cent, solution of bleaching powder. This solution helps to break up the egg masses within the cut cysts and to free the larvae from their egg shells. Thus the larvae and eggs may be counted. Using this method some difficulties have been met with. Working with three samples of filtered bleaching powder solution, several cysts were found containing a large number of unseparated eggs, thus giving counts which were too low.