The effect of sera from patients with acute visceral leishmaniasis onrnphagocytosis of L. donovani by human macrophage-like THP-l cells was examinedrnusing microscopic and radiometric methods. Leishmania antibody-positive serarnfrom clinically asymptomatic subjects, sera from treated individuals, and sera fromrnendemic and non-endemic controls were also tested. In the microscopicrnphagocytosis assay, enhanced uptake ofpromastigotes was observed at 3,6, and 24rnhrs after addition of sera from patients with active disease, individuals with treatedrndisease, or asymptomatic subjects with positive antibody titres compared to serarnfrom endemic or nonendemic controls and assays performed without human sera.rnIn some experiments, phagocytosis in the presence of sera from patients with activerndisease was greater than that observed using sera from asymptomatic individualsrnwith positive, but somewhat lower antibody titres. Enhanced phagocytosis wasrnpositively correlated with L. donovani-specific ELISA antibody titres and therncorrelation was higher when data from treated patients was excluded from thernanalysis. Measurement of phagocytosis using the radiometric method did not detectrndifferences between groups. Numbers of viable parasites 48 hrs after phagocytosisrnwere also greater using serum compared to adding no serum to the cultures.rnParasite viability was also positively correlated with the level of phagocytosis,rnsuggesting that intracellular killing was not affected by pretreatment of parasites orrnphagocytes with different groups of serum.