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-l) expression, as measured by reverse transcription and polymerase chain reaction (RT-PCR). We examined whether immunostaining for p-glycoprotein, the protein product ofmdr-1, could be used instead of RT-PCR to indicate the presence of the multi-drug-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 ofmdr-1expression quantitated with use of RT-PCR. Three osteosarcomas with undetectable levels ofmdr-lexpression by RT-PCR were negative immunohistochemically. Ten cases showedmdr-lexpression ranging from approximately 1 to 32 copies ofmdr-lmRNA/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 ofmdr-1expression. 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.