Oxidatively stressed red blood cells (RBC) and Plasmodium falciparum-infected RBC (PRBC) are avidly phagocytosed by human peripheral monocytes. Following the ingestion of PRBC the monocytes' ability to phagocytose PRBC and to generate aggressive oxidative compounds is severely impaired. In the present work the microbicidal and anti-tumour capacities of monocytes fed with diamide-treated RBC and PRBC harbouring mature (trophozoite) parasites have been investigated. The capacity of the latter, but not of the former, to phagocytose Escherichia coli and Staphylococcus aureus and to kill them, as well as ingested Candida albicans cells intracellularly, was found to be markedly impaired. Monocytes that have ingested PRBC had a significantly reduced cytostatic and cytolytic activities against a lymphoblastic tumour cell line. Monocytes fed with oxidatively stressed RBC had normal or sometimes even greater anti-tumour activities. Monocytes that have ingested PRBC showed a reduced capability to produce superoxide following stimulation with phorbol ester. Such impairment in monocyte functions may explain the reduced antibacterial and anti-tumour activities of monocytes in malaria patients, and could be consequential to their ability to resist bacterial infections and to provide means for the control of tumour development in those patients.