Fresh normal rat serum (fNRS) promoted adherence and cytotoxicity of albino rat neutrophils and macrophages to Brugia pahangi infective larvae (L3) in vitro. EDTA and not EGTA abolished the adherence activity suggesting the involvement of complement components via the alternate pathway. C3 molecules were detected on the surface of the parasite by immunofluorescence. fNRS depleted of complement by treatment with Zymosan A or of factor B by heating at 50 degrees C for 20 min, failed to promote cell adherence to the parasite. fNRS and cells from albino rat were more potent in inducing cytotoxicity to L3 than those from jird or Mastomys which may reflect the greater resistance offered by the albino rat to B. pahangi infection. In the presence of IgG and a heat labile factor, possibly complement, of immune serum, neutrophils and macrophages and to a lesser extent eosinophils adhered to and killed the larvae. Immune sera raised against microfilariae of different filarial parasites promoted cell-mediated cytotoxicity to B. pahangi L3 suggesting sharing of antigens between the two stages.