A method is described for the assay of nitrate-reductase activity in frozen and thawed cells of Chlorella. The method depends on coupling nitrate reductase with malic dehydrogenase, both enzymes and the necessary coenzyme being retained within frozen/thawed cells. Other ways than destroying permeability by freezing are considered and a short treatment of fresh cells with 1.5 M glycerol is shown to be effective. The method has been used to follow the induction of nitrate-reductase activity in Chlorella. Other enzyme activities that can be measured by similar methods, i.e. by enzymic coupling within damaged cells, are nitrite reductase and urea amidolyase.