We have investigated the presence of total, free, protein-bound and peptide-bound hydroxyproline (Hyp) in the erythrocytes of different mammals namely, camel, bovine, sheep, human, rabbit and rat. Sheep erythrocytes had the highest concentration of free Hyp, followed by camel, bovine, rat, human and rabbit. The concentration of peptide-bound Hyp was the highest in camel erythrocytes followed by bovine, sheep (p<0.01), rat, human and rabbit (p<0.001). However, human erythrocytes had the highest concentration of protein-bound Hyp followed by rat, bovine, camel, rabbit and sheep (p<0.05). Camel erythrocytes had the highest concentration of total Hyp followed by bovine, sheep, rat (p<0.01), human and rabbit (p<0.001). Results showed that the ratio of free to peptide-bound Hyp was significantly higher in sheep as compared to other examined species (p<0.001). These variations in Hyp distribution may result from differences in collagen structure and/or composition in these species. Alternatively, they may be due to the fact that the circulating concentration of free Hyp is controlled by liver Hyp oxidase which regulates the concentration of free Hyp in plasma.