Objective: To determine the role of interleukin-1β (IL-1β) in the degradation of proteoglycans and collagen by articular chondrocytes. Design: Chondrocytes were cultured in alginate beads for 2 weeks to produce extracellular matrix, followed by the addition of IL-1β for 1 or 2 days. Breakdown of extracellular matrix (with and without activation of pro-matrix metalloproteinases (MMPs) by APMA) was monitored by release of glycosaminoglycans (GAG, proteoglycans) and hydroxyproline (collagen) from the beads into the medium, and by the amount of damaged collagen in the bead. Levels of (pro)MMPs in the beads were assayed by zymography and their activity was quantified fluorometrically. Results: IL-1β induced a profound GAG release (~ 80% after 2 days at 20 ng/ml IL-1β) that was both time and IL-1β concentration dependent. Under these conditions no increase in collagen release or damaged collagen in the bead was detected. Zymography demonstrated that the synthesis of a variety of proMMPs was induced by IL- 1β, without a detectable increase of MMP-activity as measured in the activity assay. After activation of the proMMPs by APMA, a time and IL-1β concentration-dependent increase in MMP-activity was found, which resulted in almost complete deterioration of collagen already after 18 h of incubation. In the presence of APMA, GAG release from IL-1β treated beads was significantly increased from 24 to 31%. Conclusions: Our data suggest that proteoglycan and collagen degradation are regulated through different mechanisms: IL-1β induces the synthesis of active enzymes that degrade proteoglycans, such as 'aggrecanase', and inactive proMMPs. Thus, IL-1β alone is not sufficient to result in collagen-degrading MMPs. Once activated, MMPs may account for up to a quarter of the aggrecan degradation in this model.