The purpose of this study was to use a chemically defined medium to identify essential substances and optimal conditions for the liquid storage of neutrophils at 22 degrees C. Several commercially available synthetic media were evaluated: L-15, McCoy's 5a, M199, minimum essential medium, Dulbecco's MEM, NCTC135, and RPMI 1640. Proteins, glucose, pH, and neutrophil concentration were systematically studied. Neutrophils were harvested by centrifugal cell separators or phlebotomy, and their maintenance was evaluated by monitoring cell counts, dye exclusion, phagocytosis, bacterial killing, and chemotaxis. Neutrophils stored equally well in all synthetic media except L-15; however, chemotaxis was poorly maintained in synthetic media as compared with autologous plasma. RPMI 1640 was arbitrarily selected as a basal medium to evaluate storage variables. RPMI 1640 supplemented with albumin to a concentration of 1% improved chemotaxis and was equivalent to plasma as a storage medium with regard to the in vitro functions tested. Cohn fractions IV-1, IV-4, and gamma globulin were not effective substitutes for albumin. Glucose is essential for neutrophil storage; its absence from the medium correlated with poor cell function. Optimal glucose requirements depend on the cell concentration. High glucose concentrations were toxic to neutrophils; at 1,000 mg/dL, chemotaxis was depressed by 58%. Glucose utilization was dependent on the initial pH of the medium and on the cell concentration. A wide range of hydrogen ion concentrations was tolerated, and the optimum pH range was 7.2 to 7.8. Cell concentration is an important variable because it affects the pH of the medium as well as glucose utilization.