Correlation of p-glycoprotein detection by immunohistochemistry with mdr-1 mRNA levels in osteosarcomas. Pilot study. All the factors that influence prognosis in patients with osteosarcomas have not been fully determined. One reported predictor of poor outcome is increased multi-drug resistant gene (mdr-1) expression, as measured by reverse transcription and polymerase chain reaction (RT-PCR). We examined whether immunostaining for p-glycoprotein, the protein product of mdr-1, could be used instead of RT-PCR to indicate the presence of the multidrug-resistant phenotype. The sensitivity of the immunostaining was determined using KB cell sublines. For 13 cases of osteosarcoma, samples were immunostained for p-glycoprotein and the levels of mdr-1 expression quantitated with use of RT-PCR. Three osteosarcomas with undetectable levels of mdr-1 expression by RT-PCR were negative immunohistochemically. Ten cases showed mdr-1 expression ranging from approximately 1 to 32 copies of mdr-1 mRNA/cell. Of these cases, five cases contained occasional tumour cells with positive immunostaining. There was no correlation between levels of expression and the presence or number of immunoreactive cells. These results indicate that the presence of p-glycoprotein immunostaining does not reliably correlate with the level of mdr-1 expression. However it may be useful in conjunction with RT-PCR to further define different subgroups of osteosarcomas that may have different prognoses, and this is currently under investigation.