[AS PER AMENDMENT 01/29/99: Because of slow accrual for cohort 3 of Part A, concurrent enrollment will begin for Part B while cohort 3 of Part A is completed. There will be no further dose escalation in Part A. Part A will remain open to accrual until the final enrollee to Part B completes 4 weeks of study treatment. For Part B, 27 patients will be randomized with equal probability to one of two rhIL-12 doses or placebo. Semiweekly injections are given for 4 weeks.]