An antigen from Ascaris lumbricoides obtained from infected humans has been prepared which can be used for detecting immune response to Ascaris using the skin-prick test. A comparison of immune skin responses between this antigen and the commercial (Bencard) Ascaris suum antigen has been made and a RAST technique for the in vitro detection of specific IgE to Ascaris lumbricoides has been developed using the Lumbricoides antigen. A correlation between skin-prick test and RAST determinations and between RAST and total serum IgE antibodies has been demonstrated. The correlation of RAST results and serum IgE levels agreed with the division of the patients into two classes-allergic and non-allergic-on the basis of clinical criteria for allergy.