Nine patients with metastatic solid tumors were given IFN-beta by i.v. bolus injections. Six escalating doses (from 1 to 9 X 10(6) IU) followed by 6 additional injections at the dose of 9 X 10(6) IU were administered every other day (schedule A) in 3 of the 9 patients. IFN dose was also increased to a maximum of 46 X 10(6) IU, and 12 individual injections of 1, 2, 3.3, 5, 7, 9, 12, 16, 21, 27, 34 and 46 X 10(6) IU of IFN were given over a minimum of a 24-day period (schedule B) in 6 of the 9 patients. The single maximal tolerated dose ranged from 9 to 46 X 10(6) IU. The toxicity of IFN-beta given as scheduled in this study was significant but acceptable.