The protein and amino acid compositions of hair from mice carrying the naked (N) gene were compared with those of wild-type (+ / + ) mouse hair using two-dimensional polyacrylamide gel electrophoresis and amino acid analysis. In samples obtained from N/ + mice, the numbers of positions of the low-sulphur (LS), high-sulphur (HS) and high tyrosine (HT) protein spots were indistinguishable from those observed in + / + samples but the amount of HT protein was reduced in N/ + hair. In samples obtained from N/N mice the protein spots were fewer than, and perhaps different from, those in samples from the other two genotypes; all protein classes were represented but the HS and HT protein content appeared to be less than that in + / + and N/ + hair. Amino acid analyses of hair samples showed that the HS contents of + / + and N/ + hair were approximately the same, whereas the HT protein content of N/ + hair was about half that in + / + hair. N/N hair contained less half-cystine and tyrosine than + / + and N/+ hairs, but because the contents of proline, threonine and glycine were unexpectedly high, it appears that protein components of unusual composition are present in N/N hair. It was concluded that the gene at the N locus is involved only indirectly in the synthesis of the structural proteins found in mouse hair.