An experimental protocol was designed to assess humoral immune responses in animals antigenically challenged without the aid of adjuvants. The antigen selected was keyhole limpet hemocyanin (KLH). An enzyme‐linked immunosorbent assay (ELISA) was utilized as the technique to measure serum antibody levels. The procedure was tested for sensitivity by use of a known immunosuppressant (cyclophosphamide), two metals (lead and selenium), and three chlorinated hydrocarbons (polychlorinated biphenyls, pentachlorophenol, and toxaphene). KLH without adjuvant provoked an adequate humoral immune response when assessed by the ELISA. The antibody response was greater on d 15 than on d 8 following primary KLH challenge, while secondary challenge resulted in an additional 10‐fold increase in antibody levels. Cyclophosphamide suppressed the later primary response (d 15) and the secondary response more so than the early primary response (d 8). Of the S chemicals tested, 4 resulted in significantly impaired antibody levels to KLH at some time during the response. The magnitude of the immune response elicited to KLH is compared to that reported for bovine serum albumin and ovalbumin administered with Freund's adjuvant.