Recombinant proteins consisting of either the four or five amino-terminal immunoglobulin (Ig) modules of the rat neural cell-adhesion molecule NCAM or the whole extracellular part [six Ig and five fibronectin type III (F3) modules] of mouse L1 have been expressed in Drosophila S2 cells. The proteins have been purified and crystallized. The crystals of the recombinant protein containing the four amino-terminal Ig modules of NCAM diffract X-rays to approximately 4 A resolution and belong to space group P622 or P6(3)22, with unit-cell parameters a = b = 258.7, c = 182.4 A. No diffraction was observed for the other two protein constructs. This is a step towards determining the structure of multimodular constructs of cell-adhesion molecules that exhibit high structural flexibility.