MGN-3 an arabinoxylane from rice bran that has been enzymatically modified with extract from Hyphomycetes mycelia, was tested for anti-HIV activity in vitro. MGN-3 activity against HIV-1 (SF strain) was examined in primary cultures of peripheral blood mononuclear cells. MGN-3 inhibited HIV-1 replication by: (1) inhibition of HIV-1 p24 antigen production in a dose dependent manner--MGN-3 concentrations of 12.5, 25, 50, and 100 micrograms/ml showed 18.3, 42.8, 59, and 75% reduction in p24 antigen, respectively; and (2) inhibition of syncytia formation maximized (75%) at concentrations of 100 micrograms/ml. Further studies showed that ingestion of MGN-3 at concentration of 15 mg/kg/day resulted in a significant increase in T and B cell mitogen response at 2 months after treatment: 146% for PHA, 140% for Con A, and 136.6% for PWM mitogen. We conclude that MGN-3 possesses potent anti-HIV activity and in the absence of any notable side effects, MGN-3 shows promise as an agent for treating patients with AIDS.