The jird (Mongolian gerbil) is a highly susceptible experimental host for the lymphatic filarial nematode, Brugia pahangi. The chemotactic activity of serum from this host for B. pahangi infective larvae was compared in vitro to that of sera or hemolymph of a wide variety of other organisms including mammals, reptiles, fishes and invertebrates. The range of the Chemotactic Index (CI) was from 96.0 for the jird to 56.2 for a snail. An average of CI of saline control was 4.5. Significant chemotactic activity was present in many organisms, especially mammals, but was not closely related to either the phylogenetic position of the organism and to its known susceptibility as definitive host for B. pahangi. Migratory response was diminished in a consistent way by serial dilution of sera of humans, jirds and fetal bovine serum. Pre-incubation of larvae in fetal bovine serum inhibited migration, especially towards the sera of humans. Inhibition could be reversed by rinsing larvae in saline, longer rinse periods resulting in greater recovery of CI. These results are the first to suggest the activity of the specific amphid chemoreceptors in the chemotaxis of the infective larvae of B. pahangi.