The search for a synthetic medium to replace serum for maintaining cells in vitro began in the late nineteenth century and continues to this day. Ringer, Locks and Tyrode substituted physiological salt solutions augmented with glucose for serum and thereby laid the foundation for the development of defined media. As biochemical and analytical techniques have improved, more of the components in serum such as vitamins, hormones, and amino acids have been identified and incorporated into physiological salt solutions, reducing, and in some cases eliminating the concentrations of animal sera required as a medium supplement. Although there have been many modifications to the original formulas in efforts to produce fully defined media, salt solutions still play an important role in tissue culture. A salt solution's basic function, to maintain the pH and osmotic balance in the medium and to provide the cells with water and essential inorganic ions, is as valuable today as when it was first developed a century ago. Earle’s Balanced Salts is one of the balanced salts available from USBio. The selection of a balanced salt is strongly influenced by 1. type of cell, 2. type of culture monolayer, suspension, clonal, and 3. degree of chemical definition necessary. It is important to review the literature for recommendations concerning medium, supplementation and physiological parameters required for a specific cell line.