The phenomenon of multidrug resistance (MDR) in cancer cells is generally associated with P-glycoprotein (P-gp) expression and presents an obstacle to successful chemotherapy. Attempts to overcome P-gp-associated MDR using P-gp modulators, such as verapamil, have been hindered by their intrinsic in vivo toxicity. In 1991, however, Scala et al. demonstrated the alteration of P-gp function by interferon-alpha (IFN-α) in vitro at non-toxic in vivo concentrations, suggesting a basis for the use of IFN-α clinically in patients exhibiting P-gp-associated MDR. Drug resistance in B-CLL has been linked to the phenomenon of MDR, however, publications regarding this have been conflicting. The contrasting results prompted further investigation of the role of P-gp-associated anthracycline resistance and, using isolated β-lymphocytes from B-CLL patients, this investigation examined P-gp expression, function and IFN-α modulation in vitro. Optimum conditions for in vitro analysis of P-gp-associated anthracycline resistance were determined by examining the stability of the anthracycline, daunorubicin, in varying cell culture conditions. The resulting system balanced conditions affecting drug stability with those affecting cell survival. While other investigations have neglected the issue of drug stability, this study demonstrates that the instability of daunorubicin may be a critical variable determining the outcome of drug sensitivity studies. In RPMI + 2mM L-glutamine and 10% (v/v) FBS, loss of drug concentration is due to both adsorption and degradation and these experiments show that the presumed availability of drug may be over-estimated in in vitro studies. Furthermore, the degradation products might interfere with P-gp function and modulation. MDRl gene mRNA was detected in the B-cells of forty-three out of fifty B-CLL patients analysed, whereas P-gp expression, as measured by flow cytometry, resulted in only sixteen patients out of fifty-five being classed as positive (> 10% increase in staining as compared to the control). P-gp functionality and modulation studies on the B-cells of eleven patients confirmed the existence of an efflux mechanism with identical characteristics to P-gp using verapamil, the dye rhodamine 123 (rho123) and daunorubicin. Four patients were classed as functional low expressers (functional P-gp with low P-gp expression (7-10% increase in staining)), six were classed as functional high expressors (functional P-gp with high P-gp expression (20-57% increase in staining)) and one as a non-functional high expressor (non-functional P-gp with high P-gp expression (13.4% increase in staining)). Verapamil modulated rho123 efflux in all ten patients classed as P-gp functional expressors, and daunorubicin efflux in eight of these patients. However, IFN-α modulated rho123 and daunorubicin efflux in only two and one patients, respectively, even at concentrations higher than 500I.U./ml. In contrast to Scala et al. (1991), this finding suggests that at a well tolerated concentration IFN-α may not be suitable for use as a P-gp modulating agent in vivo in B-CLL, although conclusive evidence would require a larger study.