A series of inhibitors of trimming glucosidases and mannosidases were examined for antiviral activity toward HIV-1. N-butyl deoxynojirimycin (N-buDNJ) was found to be the most potent agent studied. Treatment of acutely infected lymphoid cells with 2.0 mM N-buDNJ reduced virus yield more than 90%, without affecting cell growth. Though lower concentrations of N-buDNJ (0.002-0.2 mM) did not affect HIV-1 production, there was complete inhibition of syncytia formation. Treatment of chronically infected lymphoid cells with 0.1-1.0 mM N-buDNJ resulted in no significant change in virus production, but 80% reduction of infectivity. The attenuation in HIV-1 infectivity was due at least partially to diminished binding to CD4+ lymphoid cells. Chronically infected lymphoid cells treated with 0.02-1.0 mM N-buDNJ for at least 3 days were markedly impaired in their ability to form syncytia with uninfected lymphoid cells. N-buDNJ treatment of HIV-1 infected cells resulted in both a reduction in the cell surface envelope proteins, and an increase in their apparent molecular weight. These results show that N-buDNJ can be used to impair the infectivity of HIV-1 without significant toxicity.