The mode of action of interferon-beta (IFNbeta) as a therapeutic agent for multiple sclerosis is still insufficiently elucidated. Of the known immunomodulatory effects of IFNbeta, those that seem important are its [1] effects on the production of cytokines by T helper lymphocytes, [2] antagonistic effect against some of the proinflammatory and immunoregulatory effects of IFNgamma, [3] inhibitory effect on resistance of T lymphocytes to apoptotic signals, [4] inhibitory effect on the production of matrix metalloproteases, and [5] modulatory effect on the expression of adhesion molecules and chemokines. The relative importance of each of these actions is presently unknown, and still other actions of IFNbeta may be at play. The possibility that therapeutic IFNbeta compensates for a lack of endogenous IFNbeta production is discussed.