In many cell systems, resistance to cytotoxic drugs is acquired by the amplification and/or overexpression of the multidrug resistance (mdr) gene, which codes for the glycoprotein, p170 (P-glycoprotein). Moreover, in a variety of malignant tumours there is increasing evidence of the relationship between the DNA ploidy pattern of patients and their prognosis. In this study we aimed to evaluate these two potential indicators of constitutive drug resistance in human colorectal tumours. We employed a method to quantify simultaneously, on a per cell basis, mdr gene expression (using the C219 monoclonal antibody for P-glycoprotein) and nuclear DNA content with high-resolution bivariate flow cytometry. The study was performed on a human colon-carcinoma-derived cell line (LoVo) and its doxorubicin-resistant variant (LoVo/Dx) and on tumour samples and adjacent normal mucosa from 35 untreated patients with colon cancer. The P-glycoprotein was found in both LoVo and LoVo/Dx cells with levels slightly lower in the parental than in the resistant subline (P, NS). A multi-drug-resistant specific probe for mRNA expression and Western blot assay confirmed the specificity of p170 expression. All of the colon cancer with unimodal diploid DNA distribution and all the normal colonic mucosa samples showed P-glycoprotein expression, without a statistically significant difference in median values between tumours and normal samples. Tumours with bimodal DNA distribution showed median values of P-glycoprotein expression of their hyperdiploid cell clones significantly higher than those of their diploid clones and of the tumours with unimodal DNA distribution (P less than 0.005). Our results show the feasibility of bivariate flow-cytometric analysis of P-glycoprotein expression and DNA content on clinical material and support the hypothesis that the MDR phenotype and DNA ploidy together may influence the biological behaviour of colon cancer in vivo.