Third-stage larvae of Ascaris suum cultured to the fourth stage in a chemically defined culture medium produced a substance, the 'ACF antigen', which was allergenic in the guinea-pig. When three different concentrations (3.1, 31 and 62 micrograms) of the ACF antigen were given intraperitoneally, only the highest concentration induced a primary IgE specific antibody response (1:100 titre) as determined with the passive cutaneous anaphylaxis reaction. Upon secondary exposure all concentrations demonstrated a strong IgE response (1:50,000 peak titre) with very little IgG1 activity (1:100). The secondary IgE responses began to rise on the fourth day, peaked on the sixth day and returned to relatively low levels by the fourteenth day (1:100). The intramuscular administration of the ACF antigen did not induce the extremely high titres of IgE as found with the intraperitoneal injection, but rather a low level response (1:500 peak) which did not differ greatly from the IgG1 response.