We studied the levels of matrix metalloproteinase (MMP)-2, membrane-type (MT)1-MMP, MT2-MMP, and MT3-MMP in 43 malignant pleural and peritoneal effusions using reverse transcription-polymerase chain reaction (RT-PCR) and cellular localization of MT1-MMP in 66 effusion specimens and 85 corresponding primary and metastatic tumors using messenger RNA (mRNA) in situ hybridization (ISH). In 43 effusions, MMP-2 mRNA was detected in 37, MT1-MMP in 25, and MT2-MMP in 32. Expression of MT1-MMP and MT2-MMP was found in 21 specimens; in 16 MT-MMP-positive specimens, mRNA for only 1 of 2 enzymes was expressed. MT3-MMP mRNA was not detected. High levels of MMP-2 mRNA were detected more often in effusions with high MT1-MMP and/or MT2-MMP mRNA expression. Using ISH, MT1-MMP mRNA was localized to cancer cells in 27 of 58 malignant effusions; focal signals were detected in mesothelial cells in 7 of 42. MT1-MMP was localized to tumor cells in 32 of 85 primary and metastatic solid lesions, and stromal cells expressed MT1-MMP in 3. Tumor cell MT1-MMP expression in effusion specimens did not differ from primary or metastatic lesions. MT-MMP expression in tumor cells in effusions showed no association with effusion site or tumor type using ISH and RT-PCR.