A restriction endonuclease fragment derived from a cloned portion of human genomic DNA corresponding to the myelin basic protein gene has been used to map the position of this gene by in situ hybridization to human metaphase chromosomes. Ten percent of the radioactively labeled sites observed were on chromosome 18. Eighty-four percent of the grains on chromosome 18 were located within the region corresponding to 18q22→qter. This represents a greater than 10-fold increase in labeling at this position over the background grain distribution found along all of the other chromosome