The purpose of this range finding study was to estimate the 24-h, 48-h, 72-h, and 96-h NOEC (no observed effect concentration) and ECso (i.e. that concentration which affects 50% of the test organisms within the specified time period) of Dow Corning 36 Emulsion to the green alga, Selenastrum capricornutum Printz. The results of this study will be used to design a definitive GLP study. 36 Emulsion is a water based emulsion consisting primarily of water, PDMS, and polyoxyethyleneisolauryl ether. This product is intended for use as an additive to polish formulations or as a release agent. Dow Corning 36 Emulsion is manufactured at several Dow Corning facilities world-wide. The test organisms were exposed to four concentrations, three replicates per concentration of 1000, 100, 10, and I mg/1 and a control group. In addition each concentration group included an in-group control which consisted of test medium plus test article without organisms. Exposure of organisms to the test substance was begun on June 15, 1998 and concluded on June 19, 1998. Data analyses included the OECD relative 'area under the growth curve' method and a comparison of the growth curve slopes using natural log normalized algal cell counts versus time. Linear regressions were performed on the natural log transformed growth curves. Dow Corning 36 Emulsion had no significant effect on the 96-h Selenastrum growth rate at 1, 10, and 100 mg/1. A statistically significant reduction in growth rate (11%) was observed at 1000 mg/1. Analysis of variance procedures also indicated a significantly lower (11%) 96-h growth rates for the 1000 mg/1 treatment level. Analysis of covariance found no significant differences in cell counts/ml in any treatment level for days 1,2,3, and 4 after adjusting for day 0 cell counts/ml which were significantly higher in the 1000 mg/1 treatment group. The EC50 to Selenastrum capricornutum is estimated to be >1000 mg/1.