The probability that in utero exposure to filarial antigens may influence the outcome of a subsequent infection has been investigated using a laboratory model whereby BALB/c mice are implanted with adult, female Acanthocheilonema viteae in order to generate a high-level, long lasting microfilaraemia. When infected using this procedure, BALB/c and (BALB/c x B10) F1 mice can be defined as susceptible and resistant respectively in terms of the microfilaraemia produced. By using microfilaraemic BALB/c mice as mothers, BALB/c and F1 offspring were exposed to the possibility of in utero infection. The finding of microfilariae in foetal tissues and their presence in the blood of two week old mice confirmed the transplacental transmission of parasites in both cases, BALB/c and F1 progeny born to microfilaraemic mothers failed to support a full infection from the L3 stage; similarly, progeny implanted with female worms were as sus-ceptible and resistant respectively as unexposed BALB/c and F1 controls. Spleen cells from in utero exposed, two week old BALB/c and F1 mice recognised filarial antigens in lymphocyte proliferation assays, as did their microfilaraemic mothers. In Western Blot studies, sera from such mice and from foetal, in utero exposed BALB/c mice recognised the same spectrum of A. viteae antigens as their mothers, which strongly suggests the transplacental transfer of maternal antibody. These results demonstrate that the A. viteae-mouse model may be useful in studying transplacental transmission and prenatal sensitisation in experimental filariasis.