Chloramphenicol derivatives are provided for use in the preparing of antigen conjugates for the production of antibodies specifically for chloramphenicol. Specifically, the aryl amino group is derivatized to introduce a non-oxo-carbonyl group which is used for amide formation with an antigen. The conjugate is then injected into a vertebrate for production of antisera which is isolated in conventional ways and find particular use in competitive protein binding assays.