When pH-sensitive molecules are incorporated into liposomes, drugs can be specifically released from these vesicles by a change of pH in the ambient serum. Liposomes containing the pH-sensitive lipid palmitoyl homocysteine (PHC) were constructed so that the greatest pH differential (6.0 to 7.4) of drug release was obtained near physiological temperature. Such liposomes could be useful clinically if they enable drugs to be targeted to areas of the body in which pH is less than physiological, such as primary tumors and metastases or sites of inflammation and infection.