A bioassay for cyst nematode hatching activity is described which is applicable to purification research on hatching factors. Using an easily-constructed incubation unit, based on a 96-well microtitre plate, cysts were suspended, via ''thimbles'' of wire mesh, in test samples. The use of size-graded cyst samples reduced inter-replicate variability with respect to total egg content and hatching rate. The optimised bioassay procedure used 340 mu l test volumes and replicate sizes of five cysts, and the cumulative number of juveniles hatching per cyst after 5 days was closely positively correlated with final hatch (after 31 days). This procedure significantly reduces the labour, time and space requirements of hatching bioassay.