An isoform cDNA of CCK-B/gastrin receptor was isolated from human stomach. This cDNA differed from initially cloned cDNA only in the 5'-end region and encoded a truncated isoform (delta CCK-B) in which the putative N-terminal extracellular domain of the CCK-B/gastrin receptor was completely lost. Isolation of genomic CCK-B/gastrin receptor DNA revealed that this transcript is generated by alternative usage of a novel exon, termed exon 1b. Human stomach expressed both transcripts of delta CCK-B and entire CCK-B/gastrin receptor (CCK-BR), whereas human stomach cancer cell line AGS exclusively expressed delta CCK-B transcripts. Transfection of COS-7 with delta CCK-B cDNA led to the appearance of binding sites for 125I-CCK-8. Its ligand selectivity was different from that of CCK-BR. These results suggest the molecular diversity in CCK-B/gastrin receptor subtypes.