The use of serum to supplement lymphocyte tissue culture media introduces uncontrolled variables; different serum sources, lots and concentrations can produce variability in experimental results, serum can stimulate or inhibit lymphocytes, and components of serum can react with substances whose effects on lymphocytes are being studied. To avoid these problems, we studied the ability of human peripheral mononuclear cells to survive and to respond to stimulation in an entirely synthetic medium, RPMI-1640 supplemented with L-glutamine, gentamicin, HEPES buffer and magnesium. Optimal cell concentration in this serum-free RPMI-1640 was 2.5 x 10(6) cells/ml, whereas optimal cell concentration in serum containing RPMI-1640 was 1 x 10(6) cells/ml. In this serum-free RPMI-1640, 50% of the cellular input was recovered as viable cells after 7 days of culture, which was similar to results in serum containing RPMI-1640. Mononuclear cell transformation transformation was induced by phytohemagglutinin, concanavalin A, pokeweed mitogen, streptolysin O and candida. Optimal doses of stimulants and the kinetics of the responses were similar in serum-free and serum containing RPMI-1640. This system can be used to avoid the problems inherent in systems which supplement tissue culture media with serum.