A novel class of proteins, semaphorins, nucleic acids encoding semaphorins, semaphorin peptides, and methods of using semaphorins and semaphorin-encoding nucleic acids are disclosed. Semaphorin peptides and receptor agonists and antagonists provide potent modulators of nerve cell growth and regeneration. The invention provides pharmaceutical compositions, methods for screening chemical libraries for regulators of cell growth/differentiation; semaphorin gene-derived nucleic acids for use in genetic mapping, as probes for related genes, and as diagnostic reagents for genetic neurological disease; specific cellular and animal systems for the development of neurological disease therapy.