The results obtained from 12 laboratories, dealing with six identical malignant solid tumors, assessing MDR1 phenotype using molecular techniques and immunohistochemistry have been compared. Moreover, comparisons between results of MDR1 gene expression, quantified by RT-PCR or Northern blot analysis from 10 RNA and 10 cDNA samples, were also compared between eight laboratories. Results concerning solid tumors show frequent discrepancies between the results obtained by immunohistochemistry and molecular biology techniques. Moreover, inter-laboratory discrepancies concerning immunohistochemistry techniques are observed, suggesting that the interpretation of staining is critical. Results of RT-PCR and Northern blot using RNA and cDNA show that discrepancies are less frequent than those observed using immunohistochemistry. However, Northern blot is not sensitive enough to be used in routine. The problems encountered using RT-PCR are the following: positivity threshold level, reproducibility and risks of cross-contamination.