Ghrelin is a gastric peptide hormone that has an important role in appetite control and GH release. Circulating ghrelin levels are inversely correlated with body mass index and postprandially suppressed. However, the molecular form of circulating ghrelin has not been fully characterized. We studied circulating ghrelin-like immunoreactivity (Ghr-LI) using three RIAs: one specific for only the active, acylated ghrelin (antibody G0-1) and the other two detecting both active and inactive, des-acylated ghrelin (antibody SC and the commercially available Phoenix Pharmaceuticals assay). Plasma ghrelin levels were measured in healthy subjects after a test breakfast (n = 8). Ghr-LI detected by SC and the commercial assay fell significantly at 90 and 120 min post meal (P < 0.01). G0-1 Ghr-LI decreased significantly at 30 min (P < 0.05) post meal and had returned to basal levels at 90 min. Gel permeation chromatography identified three Ghr-LI peaks in plasma. Two G0-1 Ghr-LI peaks with a molecular weight much larger than ghrelin peptide were detected. Only one Ghr-LI peak was detected by the SC and commercial RIA, at the same elution position as synthetic des-acylated ghrelin. These results suggest that the majority of circulating acylated ghrelin is bound to larger molecules, whereas des-acylated ghrelin circulates as free peptide. Assays measuring specific forms of ghrelin may be more useful in determining its physiological role than those that detect both acylated and des-acylated forms.