Rat C-6 glioma serves as an experimental model for human glioma. C-6 glioma cells carried to high culture passages in medium containing 10% fetal bovine serum when injected in vivo are unresponsive to treatment with the beta-adrenergic agonist isoproterenol and the phosphodiesterase inhibitor papaverine. When C-6 glioma cells are kept in culture in serum-containing medium, beta-adrenergic receptor density falls and, concomitantly, ability to accumulate cyclic adenosine monophosphate in response to stimulation with catecholamines declines. Responsiveness to treatment in vivo with a beta-adrenergic agonist was restored when C-6 glioma cells were cultured in serum-free defined medium prior to systemic injection into rats. Culturing of C-6 glioma cells in serum-free medium significantly increases the number of beta-adrenergic receptors when compared with C-6 glioma cells grown in serum-containing medium.