The large pepsin fragments P1 and P1X, which comprise most of the rod-like domains III of the three short arms of laminin from the mouse Engelbreth-Holm-Swarm tumor, possess full binding activity for nidogen in radioligand assays. Partial reduction (70-80%) of disulfide bonds in P1 did not reduce binding activity and allowed the separation of domain III segments originating from the A, B1 and B2 chains of laminin as demonstrated by sequence analysis. Only the B2 chain segment consisting of seven cysteine-rich repeats with similarity to epidermal growth factor showed substantial nidogen-binding activity. Further degradation of this component to an active 28-kDa fragment was achieved by a second pepsin digestion of partially reduced P1. This indicates that a major binding structure for nidogen is located within three or four cysteine-rich repeats occupying sequence positions 755 to about 920 in the B2 chain. The data also show that fragments P1 and P1X differ by the absence or presence of a large portion, domain IIIb, of the laminin A chain but are indistinguishable in nidogen binding.