In Cebus apella monkey, as with other mammalian species tested to date, two different forms of haem oxygenase, HO-1 and HO-2, are detected. With the use of cDNA fragment corresponding to HO-1 nucleotides +71 to +833, blot hybridization of RNA revealed the presence of only one HO-1 mRNA of approx. 1.8 kb in both rat and monkey liver, kidney and brain. With the use of a full-length HO-2 DNA probe, blot hybridization of RNA isolated from the same rat organs revealed the presence of two HO-2 homologous transcripts of approx. 1.3 kb and approx. 1.9 kb. The same probe detected only one message of approx. 1.7 kb in monkey organs. The rat 1.3 kb mRNA has been previously shown [Rotenberg & Maines (1990) J. Biol. Chem. 265, 7501-7506] to encode HO-2 (36 kDa). The monkey 1.7 kb mRNA and the rat 1.3 kb mRNA encode proteins with similar molecular masses and immunochemical properties as indicated by Western-immunoblotting analysis. In rat organs the relative abundance of the two mRNAs differed as follows: in the liver the 1.3 kb mRNA was by far the most abundant form; in the brain equal amounts of the two mRNAs were detected, whereas in the kidney the 1.3 kb mRNA was somewhat more abundant. The protein encoded by the 1.8 kb HO-1 mRNA in the monkey did not exhibit immunochemical reactivity with antibody to rat HO-1 in Western blotting and direct e.l.i.s.a. analysis. The data suggest that, at the primary structural level, both HO-1 and HO-2 share extensive base sequence similarity in the rat and the Cebus apella monkey. The HO-1 protein, however, appears to undergo differential post-translational and/or conformational modifications in the two species, whereas the secondary structure of HO-2 protein and antigenic epitopes are conserved among the two mammalian species.