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Background: Evidence has suggested that irradiation increases matrix metalloproteinase (MMP) expression and activation in rectal cancer to a greater level than in normal rectal mucosa. The aim of this study was to evaluate the temporal expression of MMPs in a positive control tumour cell line, HT1080 (fibrosarcoma known to express MMP-2 and MMP-9) following irradiation alone and co-cultured with human fibroblasts. Methods: HT1080 cells were exposed to irradiation at a level of 4 Gray alone, and in co-culture with the human fibroblast cell line, 46Br1G1. Flow cytometry following propidium iodide staining confirmed the position of cells within the cell cycle. Messenger RNA expression was quantified by real time polymerase chain reaction using the SYBR green dye. Protein expression was confirmed by zymography and semi-quantified using image analysis. Results: Irradiation of HT1080 resulted in cell cycle arrest. Temporal gene expression of both MMP-9 and MMP-2 showed significant increases at 24 h after irradiation (P < 0.01 and P < 0.05, respectively) which returned to baseline levels by 96 h. This resulted in increased pro-MMP-9 expression (163% at 24 h, 128% at 48 h, P < 0.05) and increased pro-MMP-2 expression (165% at 48 h, P < 0.05). However with co-culture of HT1080 and 46Br1G1 there was a significant increase (P < 0.05) in MMP-2 and 9 at the gene and protein levels and increased activation of MMP-2. No significant increase in MMP expression was observed when 46Br1G1 was irradiated alone. Conclusions: Recovery from irradiation leads to a concurrent increase in MMP production of the MMP-expressing fibrosarcoma positive control cell line but not the fibroblastic line, however, the presence of the fibroblasts were required for maximal activation of MMP-2. [ABSTRACT FROM AUTHOR]