Based on the 1-phenyl-2,3-dimethyl-3-pyrazolin-5-one series and on the 1,2-diphenyl-pyrazolidine-3,5-dione series of drugs, haptenic reagents and conjugates were synthesized and evaluated by passive cutaneous anaphylaxis in guinea pigs, and by ELISA tests using rabbit antisera against the haptens. No cross-reactivity between pyrazolinone and pyrazolidinedione haptenic reagents was found in any of the test systems. But also within each series, the animal antibodies showed rather strict specificities upon interaction with related haptens or drugs. These results are somewhat unexpected, because the haptens were used in connection with long and flexible spacer arms. Furthermore, they are not typical for certain other drug allergies. The strict specificity reduces the diagnostic potential of the haptenic reagents when used in serological tests or in skin testing.