The species specific nature of an antigenic determinant previously discovered in the scrapie form of prion protein (PrPD) from cattle, sheep and mice, was further investigated in normal prion protein (PrPC) from these and other species. This was carried out with eight different anti-peptide sera raised in rabbits against various synthetic peptides representing segments of the amino acid (aa) sequence 101-122 of ovine, bovine, murine and hamster PrP. Antipeptide serum against a peptide representing aa 107-122 of ovine PrP showed almost specific reaction and crossreacted in immunoblot with caprine and human PrP only. Antisera to the corresponding bovine sequence stained bovine and porcine PrP and to a minor extent PrP of goat, man, cat, and mink, while antiserum to the murine aa sequence reacted with rodent and monkey PrP only. In contrast, antiserum to the corresponding hamster sequence displayed a broader reactivity pattern, just like the four other anti-peptide sera to various ovine and bovine sequences. Antisera were also tested for reactivity with the pathogenic isoforms of PrP of sheep, cow, hamster and mouse and showed generally similar reactivity patterns as by using PrPC. In conclusion, the region close to the actual or putative proteinase K cleavage sites of PrP seems to exhibit high structural variability among mammalian species.