In the course of constructing a homogeneous enzyme immunoassay1 for theophylline (l, &dimethylxanthine, 1) 2 in human serum, it became necessary to prepare an analogue of the drug that could be attached to an immunogenic protein carrier. Of primary concern is the design of such an analogue (hapten) which when attached to a carrier protein and injected into animals would afford antibodies to distinguish theophylline not only from its ...