This study has demonstrated that serum obtained from animals undergoing an acute inflammatory reaction induced by an intrapleural injection of dextran is able to modulate the proliferative response to PHA of lymph node and spleen cells in vitro. This response is dependent on the concentration of the inflammatory serum and on the time of collection of the serum during the acute inflammatory process. At low concentrations of serum (0.5%) stimulatory activity was observed at all time points. At higher concentrations (1%) inhibitory activity was present in 24 and 72 h sera. These results support the previous observation that the acute non-specific inflammatory reaction is able to modify the function of different cell types. It has been suggested that both stimulatory and inhibitory factors are present and the balance between these changes during the course of the inflammatory reaction.