&NA;Caspofungin (MK 991, L 743872, MK 0991), a pneumocandin derivative, is an injectable broad spectrum fungicidal agent that is in late phase II trials with Merck & Co in the US for the treatment of serious fungal infections. It is in preclinical trials in Brazil. Caspofungin also has potential in the treatment ofPneumocystis cariniipnuemonia in immunocompromised patients and is undergoing preclinical trials in this indication. It has limited oral activity, but may be useful in the oral treatment of oropharyngeal and gastrointestinal candidiasis.Caspofungin is the first glucan synthase inhibitor to be developed as an injectable antifungal for systemic candidiasis and aspergillosis. It appears to have similar efficacy to amphotericin B but is less toxic. Poor oral bioavailability will limit it to parenteral formulations.