A function of serum in the growth medium for primary monkey kidney cells has been shown to be inhibition of proteolytic enzymes. Serum inactivates the residual trypsin remaining from enzymatic digestion of the kidneys and the proteolytic enzymes subsequently synthesized by the cells. Freshly trypsinized cells could be grown to monolayers in the absence of serum provided that they were repeatedly washed to remove residual trypsin. In the absence of serum, cell growth ceased on the 4-5th day after initiation of the culture, at which time the culture fluids became active proteolytically. When the 5th day fluids were replaced with fresh serum-free medium, cell growth was accelerated and a monolayer was attained by the 7th day. If cells were grown in the absence of whole serum but in the presence of medium containing alpha globulins or fetuin which inhibit both trypsin and cell proteases, such cultures grew as well as cultures containing serum. The sterilization of trypsin for use in digestion of tissues and cell cultures poses a serious problem. After filtration through 0.22 micron filters, trypsin preparations may still contain adventitious viruses, mycoplasma and minute forms of pseudomonas and other bacteria or bacteria-produced toxins, which pass the membrane pores. A process of purifying and sterilizing trypsin without deleteriously affecting its proteolytic activity is described.