Hydroxyproline (Hyp) content in biologial fluids is used as a parameter of collagen catabolism, especially bone resorption. In this paper, we examined the relationship between agin and serum free 4-Hyp and proline (Pro) content using a newly introduced analytical technique which is chemiluminescence determination with electrogenerated tris (2, 2'-bipyridine) ruthenium(III). Serum was collected from 50 bedridden aged people and 131 normal subjects living in Yamato-son, Amamioshima, Kagoshima. In the free 4-Hyp content of normal serum, the change in females is more gentle than that males with aging. The free 4-Hyp content of bedridden aged serum is significantly (P<0.05) elevated compared to normal aged serum. Bedridden aged females and males also show levels significantly (p<0.05) elevated in comparison to normal. These changes are due to bone resorption based on bedridden patients' atrophy. In bedridden aged people, free 4-Hyp of those with a fracture was significantly (p<0.05) elevated than in people without a fracture. No relationship between low bone density and high free 4-Hyp in serum was observed. It seems that bone resorption or bedridden atrophy may be enhanced in bedridden aged people by a fracture. In free Pro content in serum, no significant change was observed. These results show that the free 4-Hyp content in serum is useful in the clinical or biological inspection of bone resorption, especially in aged people.