This system allows the testing of volatile and unstable substances, which is problematic in static test systems. The effects of different chemical compounds were tested using growth of Chlamydomonas reinhardi as criterion. From the results the following order with increasing toxicity (EC 10 day 10) can be given: atrazine, copper sulphate, cadmium chloride, pentachlorophenol, 3,4-dichloroaniline, lindane, methyl parathion and 1,2-dichloropropane. Concerning methyl parathion and 3,4-dichloroaniline, an increase of the EC 10 values during the test was observed. The sensitivity of C. reinhardi in the flow-through test system was in many cases higher than in the static tests. Bioconcentration examinations showed a 10-times high accumulation of Cd 2+ compared with Cu 2+.