W e report that nucleic acid sequence analysis of a full-length cDNA clone for a rabbit serum amyloid A (SAA)-like protein has identified this protein as more closely related to SAA3 than to SAA1. SAA3 induced collagenase synthesis in rabbit synovial fibroblasts, and immune IgG raised against this SAA protein abrogated the induction. Using antisera to immunoprecipitate biosynthetically labeled 3H-SAA and 3H-collagenase from culture medium, we compared the levels of SAA and collagenase synthesized by cultures of rabbit fibroblasts at early passage (passages 3-6) with those synthesized by late passage cells (passage 16). Comparatively high levels of both proteins were produced constitutively by fibroblasts at low passage. With increasing passage, levels of both proteins drop so that by passage 16, constitutive production of SAA and collagenase was only approximately 15-20% that of passage 3 cells. Cells at low passage could be readily stimulated with phorbol myristate acetate (PMA) or interleukin 1 (IL-1) to synthesize increased amounts of both SAA and collagenase. In passage 5 cells treated with PMA, we detected increased SAA mRNA by 1.5 h and collagenase mRNA by 5 h. However, older passage cells were more refractory to stimulation and required longer induction times. We suggest that SAA3 may be expressed by fibroblasts at sites of acute inflammation or injury, and that elevated levels of SAA3 may signify "activated" fibroblasts which are already producing increased amounts of collagenase constitutively and which are predisposed to further stimulation.