A new inhibitor of human immunodeficiency virus (HIV) has been isolated and purified to homogeneity from the seeds and fruits of the Momordica charantia. This compound, MAP 30 (Momordica Anti-HIV Protein), is a basic protein of about 30 kDa. It exhibits dose-dependent inhibition of cell-free HIV-1 infection and replication as measured by: (i) quantitative focal syncytium formation on CEM-ss monolayers; (ii) viral core protein p24 expression; and (iii) viral-associated reverse transcriptase (RT) activity in HIV-1 infected H9 cells. The doses required for 50% inhibition (ID50) in these assays were 0.83, 0.22 and 0.33 nM, respectively. No cytotoxic or cytostatic effects were found under the assay conditions. These data suggest that MAP 30 may be a useful therapeutic agent in the treatment of HIV-1 infections. The sequence of the N-terminal 44 amino acids of MAP 30 has been determined.