A new method was developed to detect the activity of the Na+-H+ exchange system as changes in cell volume. The cytoplasmic pH of isolated cells in suspension was lowered by incubation in Na-propionate medium, due to permeation of the protonated acid. This resulted in activation of Na+-H+ countertransport, measurable either as a Na+-dependent alkalinization or as an increase in 22Na+ uptake, both of which are amiloride sensitive. The continued operation of the antiport on prolonged exposure to Na-propionate results in a considerable increase in Na+ (and presumably propionate-) content. This is accompanied by an osmotic water shift and cell swelling, detectable by electronic sizing. This method was used to investigate the presence of the Na+-H+ exchanger in human platelets, neutrophils, lymphocytes, and monocytes as well as in cultured cell lines of B and T lymphoblasts and of macrophages. All these cell types displayed an amiloride-sensitive swelling when suspended in Na-propionate media. The results suggest the ubiquity of the Na+-H+ exchange system in the plasma membrane of nucleated mammalian blood cells.