The single-celled green alga Chlamydomonas eugametos was found to be well suited for detecting growth inhibitor activity of chemicals. Tests consisted of solubilizing technical grade compounds in 20-ml aliquots of a 1 × 105 cells/ml zoospore culture. Commercially available growth inhibitor herbicides significantly inhibited synchronous cell population increases within 48 h at concentrations ranging from 1 × 10−4 to 1 × 10−7M. Examples of compounds that inhibited Chlamydomonas more than 50% were butylate (S-ethyl diisobutylthiocarbamate) and diallate [S-(2,3-dichloroallyl)diisopropylthiocarbamate] at 1 × 10−4M, CDEC [2 chloroallyl diethyldithiocarbamate] and propham (isopropyl carbanilate) at 1 × 10−5M, alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] and trifluralin [α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine] at 1 × 10−6 M, and bifenox [methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate] at 1 × 10−7M. Ethanol, dimethylsulfoxide (DMSO), or acetone can be used to solubilize herbicides in the aqueous medium. DMSO and ethanol are not detrimental to the cells if the concentration is kept at 1% v/v or less.