Cytotoxicity was measured in vitro for 8 ethylene oxide/propylene oxide copolymers (EO/PO copolymers) using lactate dehydrogenase release from cultured mammalian cells as the endpoint. Three cell types were used in these assays: Chinese hamster ovary cell line (AS52), rat lung epithelial cell line (LEC), and freshly isolated rat alveolar macrophages (RAM). A range of cytotoxicity was seen with toxic effects observed from 20 to > 20,000 micrograms/ml. The same relative order of toxicities were observed for all 3 cell lines although RAM cells appeared to be somewhat more sensitive. The in vitro cytotoxicity, as measured by LDH release and microscopic observations of the cells, correlated poorly with the in vivo inhalation toxicity. The most lethal compounds following acute inhalation (UCON 50-HB-5100 and UCON 50-HB-2000) were among the least toxic in the in vitro cytotoxicity screen. Conversely the 2 compounds which were the most toxic in vitro (Pluronic 17 R1 and Pluronic L64) did not produce any unusual degree of toxicity in inhalation studies. The results of these experiments indicate that these in vitro mammalian cell assays will not be useful, at least for these classes of chemistry, in prediction of in vivo inhalation toxicity.