The structural relationship between human alkaline phosphatase isoenzymes from placenta, intestine, liver, bone and kidney was investigated by lectin-binding affinity chromatography. In addition, antibody-binding sites of the enzymes were studied using monospecific antisera against each of the isoenzymes. Evidence is offered for the existence of three classes of alkaline phosphatases: the placental isoenzyme, the intestinal isoenzyme and the liver-bone-kidney-type-isoenzyme: 1. A high affinity to bind to concanavalin A and lentil lectin characterizes the placental isoenzyme in contrast to the other isoenzymes. The intestinal isoenzyme remains totally unbound. The liver-bone-kidney-isoenzyme demonstrates a microheterogeneity with bound and unbound parts. A small unbound fraction can be detected in the placental isoenzyme, also when lentil lectin is used. 2. The placental isoenzyme and the isoenzymes purified from liver, bone and kidney are bound by wheat germ lectin-Sepharose, but not the intestinal isoenzyme. All isoenzymes are eluted as a homogeneous peak. 3. Using Helix pomatia lectin-Sepharose, all isoenzymes are unreactive except a minor fraction of kidney alkaline phosphatase. 4. Antibodies to the placental isoenzyme show a partial cross-reaction with the intestinal isoenzyme, but can be obtained monospecific after absorption. 5. Antibodies to the intestinal isoenzyme show a partial cross-reaction to the placental isoenzyme, respectively, but are monospecific again after absorption. 6. Antibodies to liver, bone or kidney isoenzymes show a complete cross-reaction, but are unreactive with the placental and intestinal isoenzyme; after absorption with a heterologous isoenzyme of this group, no further reaction can be demonstrated with any of the three isoenzymes. Thus, lectin-binding affinity identifies the same isoenzyme classes by their carbohydrate parts, as antibodies presumably do by the protein parts of the isoenzymes. Furthermore, lectin-binding affinity demonstrates a microheterogeneity of the placental isoenzyme with lentil lectin-Sepharose and of the liver-bone-kidney-type-isoenzyme with different lectins.