ExtractHistochemical and ultrastructural consequences of pH variations in growth media were studied using three normal human fibroblast strains. Histochemical staining for acid phosphatase indicated increased amounts of enzyme activity in cells grown at higher pH. The ultrastructural consequences of an increase in growth medium pH was dramatic. A picture resembling storage disease developed in these cells within a few days. The cells became filled with bodies which could roughly be divided into two types. Type I had a homogeneous matrix, stained slightly with ruthenium red, was usually not membrane bound, and exhibited no acid glycerophosphatase activity. The nature and origin of these bodies is unclear. Type II bodies contained whorles of membranes and inhomogeneous matrix which was acid phosphatase positive but did not stain with ruthenium red. This type was always membrane bound and qualifies as lysosomes. Normal cells thus grown at higher pH were morphologically similar to cells cultured at neutral pH from patients with I-cell disease. These morphologic changes did not affect cell viability or cell growth.SpeculationA morphologic picture of severe storage disease can be induced in normal fibroblasts simply by increasing the medium pH. This is in agreement with our earlier observation that the lysosomal function of mucopolysaccharide degradation is progressively inhibited as pH of the growth medium increases. When attempting to diagnose genetic disease in cultured cells it is important to realize that intracellular lysosomal function is sensitive to extracellular pH. The observation of such pH sensitivity might also be a useful tool in studies on the general nature of lysosomal function.