It is widely accepted that specific inhibitors of the HIV-1 aspartic protease might be interesting chemical leads to develop effective therapeutic agents for the treatment of AIDS12).In the course of a screening program to discover new inhibitors of HIV-1 protease from microbial fermentation broths3), we have isolated the novel molecules GE20372 factors A (1) and B (2) shown in Fig. 1. These microbial products are tetrapeptide derivatives structurally related to the previously described microbial alkaline protease inhibitors (MAPI)-a (3) and ft (4)4~6). Here we describe the isolation, structure elucidation and inhibitory activity of GE20372 A and B.