Our previous studies on the mechanism of cytotoxic action of the anti-tumour drug adriamycin (ADR) indicated that this anthracyclinic antibiotic strongly modified the molecular architecture of the plasma membrane of human erythrocytes, presumably becoming incorporated within both lipid layers. In order to verify this hypothesis, electron energy-loss spectroscopy (EELS) has been used to compare the P content in control and ADR-treated erythrocyte ghosts. EELS measurements allowed us to reveal a significant reduction in the P/C ratio in erythrocyte ghosts after ADR treatment. This finding seems to reflect a phospholipid 'dilution' produced by the incorporation of the drug molecules in the membrane layers. A structural model of the ADR-membrane interaction is proposed.