Antisera raised against synthetic sulphated cholecystokinin (CCK 26-33) (n = 4) and against 30% pure porcine CCK (n = 11) were characterized by the use of different labelled CCK peptides. CCK 39 and CCK 26-33 were coupled to 125I by the chloramine-T method, while CCK 33 was conjugated to 125I-labelled hydroxyphenylpropionic acid-succinimide ester (Bolton-Hunter reagent). Antisera raised against CCK 26-33 bound to 125I-CCK 26-33 only. Of the antisera raised against 30% pure CCK, 2 bound to all 3 labels, 4 to 125I-BH-CCK 33 and 125I-CCK 26-33, 3 to 125I-CCK 39 and 125I-BH-CCK 33, 1 to 125I-CCK 26-33 only and 1 to 125I-BH-CCK 33 only. The antibodies reacting with 125I-CCK 26-33 bound also to 125I-gastrin 1-17. Different CCK labels bound to different binding sites in the same antiserum (antibody heterogeneity). The pattern of reactivity of the antiserum to CCK peptides was dependent on the type of label used. With these different labels, antibodies specific for CCK 39, for CCK 33 and CCK 39, for sulphated forms of CCK, and for all CCK peptides and gastrin could be detected. It is concluded that antisera raised against CCK should be characterized by use of different labelled CCK materials.