The effect on a range of biologic responses of interferon-beta serine (IFN-beta ser), administered by either the sc or the iv route, was examined in 16 patients. Despite the absence of IFN in the serum of 13 of 16 patients after sc administration, biologic changes associated with IFN administration occurred. Significant increases in peripheral mononuclear cell surface proteins were evident. Monocyte human leukocyte antigen-DR (HLA-DR) showed a 23% increase in mean fluorescent intensity (P = .04) and a 9% increase in percentage of positive cells (P = .02); lymphocyte OKT10 had an 11% increase in percentage of positive cells (P less than .0001) and a 26% increase in mean fluorescent intensity (P = .002). Natural killer cell activity against the Change target increased by 125% (P = .004). Intracellular activity of 2',5'-oligoadenylate synthetase increased 297% at 24 hours and 226% at 48 hours (P less than .0001). Significant increases in serum concentrations of beta 2 microglobulin (24% at 24 hr and 27% at 48 hr, P less than .0001) and neopterin (85%, P = .0001 and 165%, P = .00001) were observed. These alterations after sc administration were similar quantitatively to those resulting from the same dose of IFN-beta ser given iv. Thus, serum IFN concentrations did not have to be measurable for IFN-beta ser to exert biologic activity. The different effects of two dose levels, 45 X 10(6) IU and 180 X 10(6) IU, also were compared independent of route. The higher dose resulted in greater increases over baseline of 2',5'-oligoadenylate synthetase activity (344% vs. 145% at 24 hr; 231% vs. 83% at 48 hr) and serum neopterin concentrations (185% vs. 99% at 24 hr; 271% vs. 153% at 48 hr). For all the other parameters, there was no significant difference between the two doses.