We have reviewed the structure and properties of the neural protein 1B236/MAG. This molecule consists largely of five Ig-like domains separated from its carboxyl terminal tail by a single membrane-spanning region. Two forms of the protein differ in the length and sequence of the carboxyl terminus: these are encoded by alternatively spliced mRNAs that are differentially expressed during postnatal neural development. The Ig-like domains of 1B236/MAG are unusual in having structural similarities to Ig V domains but with short Cys-Cys distances characteristic of C domains. Several other Ig-like molecules exhibit this structural feature, including the cell adhesion molecule N-CAM, which is most closely related in sequence to 1B236/MAG. We have proposed 1B236/MAG as the prototype for this subgroup of the Ig family and offer a model for this type of Ig domain structure. 1B236/MAG probably acts as a cell adhesion molecule to mediate interactions between cells in a fashion similar to that proposed for N-CAM. In particular, 1B236/MAG may be involved in interactions between myelinating oligodendrocytes or Schwann cells and axons or between adjacent layers of myelin membrane during the process of myelin compaction. It is most likely that the homophilic or heterophilic interactions of 1B236/MAG occur through binding to the Ig-like domains. The structure of 1B236/MAG is therefore quite consistent with its proposed function and may serve as the model for this class of cell-cell interaction molecules. One would predict, for example, that the neuron-glia cell adhesion molecule Ng-CAM, also known as NILE or L1 (Bock et al. 1985, Friedlander et al. 1985), which mediates interactions between neurons and glial cells, would have a very similar structure to those of N-CAM and 1B236/MAG. In addition, the carboxyl terminal tails of the 1B236/MAG proteins may also be involved in interactions with cytoskeletal components, during membrane vesicle transport through the glial cytoplasm during myelination or through neuronal axoplasm or cytoplasm. The availability of full-length cDNA clones of 1B236/MAG mRNAs with the ability to express these products in vitro will enable the structure and interactions of 1B236/MAG to be tested in detail.