Objective. The effects of doxycycline were tested in an in vitro system in which the cartilages of embryonic avian tibias are completely degraded. Methods. Tibias were cultured with 5, 20, or 40 μg/ml doxycycline. Control tibias were cultured without doxycycline. Conditioned media and tissue sections were examined for enzyme activity and matrix loss. Results. Cartilages were not resorbed in the presence of doxycycline, whereas control cartilages were completely degraded. Collagen degradation was reduced in association with treatment with doxycycline at all doses studied. Higher concentrations of doxycycline reduced collagenase and gelatinase activity and prevented proteoglycan loss, cell death, and deposition of type X collagen in the cartilage matrix; in addition, treatment with doxycycline at higher concentrations caused increases in the length of the hypertrophic region. Conclusion. The effects of doxycycline extend beyond inhibition of the proteolytic enzymes by stimulating cartilage growth and disrupting the terminal differentiation of chondrocytes.