Investigations to characterize the in vitro reaction of serum-mediated leucocyte-adherence to microfilariae of Wuchereria bancrofti were carried out in Tanzania. The adherence reaction took place within one hour and at least two serum factors were involved: a heat-labile factor, present also in normal serum (probably complement), and a more heat-stable factor, present in positive serum only (probably an antibody). Neutrophils and eosinophils were involved in the reaction proportionately to Their numbers in the cell solutions used; the reaction killed significantly more microfilariae, than did adherence-negative tests. Microfilariae could be used for at least one week after their isolation from the donor, but a significant decrease in the percentage of reactive microfilariae occurred within 16 hours of isolation. A high degree of species specificity was observed when positive sera were tested against other nematodes. The involvement of complement and the speed with which the reaction took place is in contrast to the results of similar studies from India, and may indicate a difference in the parasite in the two continents.