Microinjection is an effective and reproducible method for introducing membrane-impermeable molecules into cells in culture. The method has been in existence almost as long as there have been microscopes to observe the process. Inducible nitric-oxide synthase (iNOS) plays a critical role in the clearance of intracellular pathogens through NO production. iNOS in macrophages is negatively regulated by the SPSB proteins, which drive its proteasomal degradation.[1] Inhibiting the SPSB-iNOS interaction prolongs the intracellular lifetime of iNOS, which enhances the clearance of chronic infections caused by bacteria and parasites.[1] In this study, microinjection was employed to deliver a redox-stable peptide inhibitor of the SPSB2-iNOS interaction (CP2, KD 21 nM)[2] intracellularly to assess possible toxicity associated with this peptide. The co-injection of peptide and Cy5-labelled BSA in AD-293 cells showed no significant mortality induced by the tested conditions (2.7 and 273 μM peptide). The results of these studies are valuable starting points in evaluating toxicity early in the development of cell-impermeable synthetic peptides as potential therapeutics. More potent peptide inhibitors are being developed[3] in parallel with our ongoing efforts to deliver them to cells using a range of strategies.