A full-length clone for the human proteolipid protein (PLP) was isolated from a cDNA library constructed from poly(A)+ RNA isolated from fetal spinal cords obtained at 15-24 weeks of conceptional age. The sequence of the human PLP cDNA was determined, and the deduced amino acid sequence was found to be identical with that of rat PLP. Comparison of human and rat PLP cDNA clones indicated that the coding regions retained 97% homology and that there were also other areas of conserved sequence. The human 5'-untranslated region was 93% homologous to that of the rat. The 3'-untranslated region was, overall, 73% homologous to that of the rat with areas containing greater than 84% homology in the first 400 and last 200 nucleotides. The most variability within the 3'-untranslated region occurred between nucleotides 2,000-2,500, where homology with the rat cDNA dropped to 55%. Expression of PLP in the human spinal cord between 11 and 23 weeks after conception was examined and compared with the expression of the myelin basic protein (MBP). RNA was isolated from pooled human spinal cords obtained at three periods of development: 11-14 weeks, 17-19 weeks, and 21-23 weeks. Northern blot analysis revealed a 3.2-kilobase (kb) PLP mRNA that was present at higher abundance in the 21-23-week spinal cord RNA than in the 17-19-week or the 11-14-week samples. The 17-19-week RNA sample also contained a PLP-hybridizing band at 2.2 kb which may possibly have arisen by utilization of alternative polyadenylation signals. Messenger RNA for MBP was detectable at 11-14 weeks but was readily evident in both the 17-19- and 21-23-week age groups. Immunoblot analysis of whole spinal cord homogenates indicated that polypeptides for MBP preceded the appearance polypeptides for PLP by 3-4 weeks.