Tetraspanins are transmembrane adaptor proteins involved in the regulation of various fundamental cellular processes. For a number of malignant diseases, the level of expression of members of the tetraspanin family was found to correlate with tumor cell invasiveness, ability to form metastases, and poor clinical outcome. We describe the exact quantification of mRNAs coding for the tetraspanins CD9, CD63, CD82 and CD151 expressed by mammary carcinoma-derived cell lines that were classified as invasive or non-invasive according to their ability to penetrate collagen-fibroblast gels in vitro. The mean of beta2-microglobulin-normalized expression of CD9 was about 10-fold higher than the mean calculated for CD63 and about 20-fold higher than expression of CD82 and CD151. Direct comparison of tetraspanin expression of invasive and non-invasive cell lines with the Mann-Whitney test revealed a significant correlation for CD63. Grouping of cell lines in relation to threshold values of expression resulted in significant correlations for CD63 (Fisher's exact test p=0.004) and CD151 (p=0.02) but not for CD82 (p=0.065) and CD9 (p=0.168). Expression of CD9, C63 and CD151 was found to be coupled whereas CD82 was expressed independently. This highly significant association points to common mechanisms of gene regulation for this subgroup of tetraspanins. We showed that on basis of absolute amounts of tetraspanin mRNAs, at least in vitro invasiveness is clearly predictable. Our results support the assumption that downregulation of tetraspanins in breast cancer cells is an important step of tumor progression to more malignant phenotypes and underline their important role as mediators in multimolecular membrane protein complexes regulating cell adhesion and migration.