Chlorophyllase (Chlase) actívity measured as the in vitro degradation of chlorophyll (Chl) was variably enhanced by various heavy metals (Cd2+, Pb2+, Mn2+, Co^"*", Ni2+ and their mixture). The en2yme actívity on Chl a was higher than that on Chl b. Chlase isolated from Chlorella fusca oř Kirchneriella lunaris responded differently to the heavy metals. The inclusion of Ca2+ in the assay media affected the enzyme actívity. While some concentrations were stimulatory, others were inhibitory depending on partícular heavy metal, the algal species, and the Chl a oř b.