Complementary and genomic DNA clones corresponding to the human C-reactive protein (CRP) mRNA and structural gene have been analyzed and compared. Nucleotide sequencing of the coding regions of both cDNA and genomic DNA revealed an additional 19 amino acid peptide not described in the published CRP amino acid sequence. The CRP gene contains a single 278 base pair intron within the codon specifying the third residue of mature CRP. The intron contains a repetitive sequence (GT)15G(GT)3 which is similar to structures capable of adopting the Z-DNA form. A comparison of CRP coding and amino acid sequences with those of serum amyloid P component revealed striking overall homology which was not uniform: a region of limited conservation is bounded by two highly conserved regions.