We have identified a synthetic peptide derived from the BZ-chain of mouse laminin, Arg-Asn-Ile-Ala-Glu-Ile-Ile-Lys-AspBe (p20), which simulates the neurite outgrowth-promoting activity of the native molecule. In organotypic cultures, neurons from newborn mouse brain or embryonic peripheral nervous system responded by extensive neurite outgrowth for native laminin or the peptide p20 in the culture medium. If rat cerebellar neurons were grown on laminin, l-5 FM (l-5 pg/ml) of peptide p20 in the culture medium competed with laminin and inhibited neuronal attachment and neurite outgrowth, whereas higher concentrations (> 50 FM; > 50 pg/ml) had a specific neurotoxic effect. When peptide p20 was used as the culture substratum, neurite outgrowth in cerebellar cultures was up to 60% of that seen on native laminin. Our results indicate that a neurite outgrowth-promoting domain of laminin is located in the a-helical region of the BZ-chain, and is active for both central and peripheral neurons.