Delivering large molecules into the cytosol of animal cells without damaging the cells has been one of the toughest challenges in biology. Endo-Porter is a weak-base amphiphilic peptide that was designed to deliver morpholino antisense oligomers and other non-ionic substances into the cytosol/nuclear compartment of cells by an endocytosis-mediated process that avoids damaging the plasma membrane of the cell. This prevents the loss of vital cell contents and the attendant high cell toxicity typical of most delivery systems. To deliver a substance into cells simply add that substance to the medium covering the cells, followed by addition of the pre-formulated Endo-Porter solution and swirl to mix. While most delivery reagents begin to show serious toxicity after just a few hours and are relatively ineffective in the presence of serum, delivery with Endo-Porter may be continued for 24 hours, or even 48 hours, without undue toxicity, and Endo-Porter is particularly effective in the presence of the 10% serum commonly used with cultured cells. The mechanism of delivery, entails rapid adsorption of Endo-Porter to cell surfaces, but this adsorption to the cell surface does not damage the plasma membrane. The membrane-bound Endo-Porter is rapidly endocytosed, along with any substances present in the medium (ie., the cargo one wishes to deliver). Upon subsequent acidification of the endosome (a natural process) the Endo-Porter contained within that endosome is converted to its poly-cationic form, which acts to permeabilize the endosomal membrane. This acid-induced permeabilization of the endosomal membrane allows any co-endocytosed cargo to pass from the endosome into the cytosol of the cell. This paper describes the basic design strategy used to develop Endo-Porter, test systems used to guide that development, and the effects of various structural parameters, including size and composition of the lipophilic face, size and composition of the weak-base face, and the relationship between peptide length and delivery efficiency in the presence of serum.