We have cloned and characterized cDNAs coding for two variants of Xenopus laevis H5 histone protein (previously called H1s). cDNA was synthesized from RNA of immature erythrocytes in a single reaction using a modification of the method of Gubler and Hoffman [Gene 25 (1983) 263-269], and blunt-end ligated into the HincII site of the phage vector M13mp9. Immunological screening with a polyclonal antibody yielded two clones expressing H5 peptide. Sequence characterization revealed that both clones contained partial cDNA inserts and that the smaller 340-bp clone initiated reverse transcription within the coding region, at a site rich in adenine. Rescreening of the cDNA bank by nucleic acid hybridization produced eleven additional H5 clones, one of which coded for a second variant of H5. These two variants, called XLH5A and XLH5B, are very similar in sequence and code for proteins of 195 and 193 amino acids, respectively, which may be the H1D and H1E variants observed previously. XLH5, avian H5 and human H1O share identity at both nucleotide and amino-acid sequence levels. Further, the XLH5-coding mRNA is likely polyadenylated and lacks the highly conserved, 23-nucleotide dyad symmetry element found within the 3' untranslated regions of most histone-coding mRNAs.