The family of Helicobacter cysteine-rich proteins (Hcp) constitutes one of the largest protein families that are specific for proteobacteria from the delta/epsilon subgroup. Most of the proteins belonging to this family have so far only been recognized on the genome level. To investigate the expression of Hcp proteins in vivo we analyzed titers of antibody against HcpA (HP0211), HcpB (HP0336), HcpC (HP1098), and HcpE (HP0235) in sera from 30 Helicobacter pylori-positive individuals and in a control group of six H. pylori-negative individuals. Significantly higher titers of antibody were observed for H. pylori-positive individuals (P < 0.00005). The highest and lowest titers were observed for HcpC (Delta mean = 1.06) and HcpB (Delta mean = 0.333), respectively. There is a clear correlation among anti-HcpA, -HcpC, and -HcpE immunoglobulin G titers in H. pylori-positive individuals (correlation > 0.7), but there is only a weak correlation for HcpB (correlation < 0.4). These results confirm that Hcp proteins are expressed by H. pylori under natural environmental conditions and that these proteins are recognized by the immune system of the host. The observed correlations are in agreement with the expected distribution of Hcp proteins among H. pylori strains. HcpA, HcpC, and HcpE are present in the genomes of strains 26695 and J99, whereas HcpB is absent from most strains. Since Hcp proteins are specific for H. pylori, immunological assays including Hcp proteins might be of value to detect H. pylori infection and perhaps to distinguish among different groups of H. pylori-positive patients.