A clinical and laboratory trial was designated to test the value of a potentially active pool of transfer factor (TF) given for a period of 3 months, at weekly intervals, in 27 relapsing MS patients and controls. The pool of TF was extracted from peripheral lymphocytes of 36 normal individuals presensitized with DNCB as marker. It was biologically capable of transferring DNCB sensitivity to MS recipients and did not show any toxicity. Clinically, a slight but not significant improvement of the functional and disability indices was observed in the TF group over a period of 1 year, while both indices increased in the control group. The treatment had no influence on the number of relapses and/or on sensory and visually evoked potentials, axial tomography and electronystagmography. In laboratory tests, a significant difference was found in the total CSF protein (P less than 0 . 05) and IgG (P less than 0 . 01) levels in the two groups studied; both values decreased or were stabilized in the group receiving TF, while they increased in the control group. Whether or not these slight clinically and biologically beneficial effects were due to the high dose of TF given or to its biological activity remains to be established. This pilot study suggests that a more appropriate answer regarding TF in MS might be obtained by using biologically active material, given for longer periods of time, at a closer interval and in a larger number of patients.