Naphthaleneacetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), and 2-chloro-4-ethylamino-6-isopropylamino s-triazine (atrazine) were applied to Chlorella pyrenoidosa in an attempt to characterize mathematically the growth inhibition responses obtained. (Chlorella was found to serve well as a test organism.) Inhibition results induced by NAA and 2,4-D satisfactorily supported the hypothesis inherent in the kinetic interpretation. Results with atrazine were most satisfactorily interpreted by a probit analysis which showed large differences in rate of toxicity between high and low concentrations. This difference is thought to be responsible for the poor fit to a straight line in the reciprocal plots of the kinetic analysis. It is assumed that growth inhibition by atrazine resulted from a blocking of the photosynthetic process.