The recent development of serological techniques for the detection of commonly abused drugs has provided rapid and highly sensitive diagnostic test procedures. In view of the pressures for drug testing on an ever-increasing scale, many of these methods have found their way into toxicological laboratories with little but the promotional literature for backing. Therefore, it is not surprising that confusion exists with regard to the proper use and interpretation of the serological tests and their relation to the more conventional chromatographic and fluorometric procedures. The far-reaching consequences of a false diagnosis of drug abuse demand that those engaged in testing not only master the arts of the various techniques but that they also be fully informed about basic principles, limitations, and appropriate interpretations. While this presentation will concern itself primarily with one serological test, namely hemagglutination-inhibition (HI) a more general consideration of serological testing appears unavoidable in view of that which has just been discussed.