Levcycloserine is a general inhibitor of pyridoxal 5'-phosphate (PLP)-dependent enzymes. It is an excellent inhibitor of threonine deaminase and serine palmitoyltransferase. Levcycloserine is a ceramide synthesis inhibitor. Levcycloserine is a selective antitumoral agent for neuroblastoma and medulloblastoma cells with the ability to reduce expression of tumour associated gangliosides. In vivo experiments suggest that levcycloserine may be effective drug for treatment of neuroblastoma and medulloblastoma. Levcycloserine interferes with the life cycle of HIV. Levcycloserine selectively down-modulated CD4 expression without affecting the expression of CD3 and CD8. Levcycloserine also inhibited T cell mitogen responses without affecting IL-2 production. Selective inhibition of CD4 by levcycloserine together with its antiviral effects may offer a novel approach for interfering with HIV cell binding and infectivity.