Beige mice, osteopetrotic from incorporated mi/mi genes, and their skeletally normal siblings were X-irradiated and converted to radiation chimaeras by rescue with normal CBA bone marrow. The procedure produced rapid cure of the osteopetrosis. Electron micrographic measurement of the decline in giant lysosomes of the host mice indicated that the longevity of the host's osteoclasts was up to 30 days in the osteopetrotic and 40 days in the skeletally normal mice. In each case the decline was roughly exponential with half-times of about 6 and 8 days respectively. Monogenetic and perhaps polygenetic and teleological factors are invoked. Absence of a latent period before decline in the score seen in the companion paper is attributed to the presence of partially mature osteoclast precursors in bone marrow, not present in spleen.