In vivo bioactivity of microspheres. IL-1β injection alone resulted in a significantly lower NP GAG content compared to intact controls. For the combined IL-1β and IL-1ra microspheres injection, NP GAG content was not significantly different from intact controls. For the saline sham injection, NP GAG content was lower than intact controls, but not significantly, and not significantly different from IL-1β alone or combined IL-1β and IL-1ra microspheres. *p < 0.05 vs. intact controls; +p < 0.05 vs. IL-1β alone.