An estimated 19 000 deaths and $3-4 billion in health care costs per year in the United States are attributed to methicillin-resistant Staphlococcus aureus (MRSA) infections. Certain conjugated phenylene ethynylene (CPE)-based polymers (PPE) and oligomers (OPE) have been demonstrated to exhibit dark and light-activated antimicrobial activity. Until recently, the relative cytotoxicity of these PPEs and OPEs toward mammalian cells haas been unknown, limiting the applications for which they may be used (e.g., reducing and/or preventing the spread of untreatable bacterial strains). In this work, we examine the toxicity of CPEs to mammalian cells using cytotoxicity assays of cellular monolayers. Eight CPEs, two PPEs and six OPEs, were selected for these studies based on their biocidal activity, and diversity of repeat unit number and functional groups. Briefly, two cell types were exposed to CPEs at concentrations ranging from 1-100 ug/mL for 24 h. We find that concentration largely determines the resulting viability of cells, although at intermediate concentrations (5-10 ug/mL), the effect of light on light-activated compounds is very important. Furthermore, we find that the longer-chained compounds are cytotoxic at much higher concentrations, and therefore have the widest range of concentrations available for potential applications.