Male jirds (Meriones unguiculatus) were inoculated sc with 100 infective larvae of Brugia pahangi. After 16 weeks, the animals were reinoculated with a comparable number of organisms. Blood eosinophil responses during the 5 weeks subsequent to this attempt to reinfect were much lower than those of comparable naive animals, while the response to a heterologous infection (Toxocara canis) was comparable to that of controls. Mebendazole was given to infected animals for 2 weeks beginning 5 weeks (prepatent) or 16 weeks (patent) after infection. At comparable intervals after drug administration, the animals were reinoculated with infective larvae and the blood eosinophil response was measured over a 5 week period. The response in the animals treated during the prepatent period was higher than the untreated infected controls. Treatment during the patent period had no demonstrable effect. Jirds made artificially microfilaremic by intravenous inoculation of viable filaria before and after the standard infecting dose had a low eosinophil response to infective larvae. A primary experience of jirds with the microfilariae of B. pahangi evokes an eosinophil response. Subsequent inoculation of larvae did not produce a comparable response.