HAM56 (human alveolar macrophage) is a monoclonal antibody that reacts with macrophages and endothelial cells. There has been controversy as to its usefulness in differentiating adenocarcinomas of ovarian or gastrointestinal origin. The aim of this study is to test the specificity of HAM56 in identifying the origin of metastatic adenocarcinomas. Ninety-two adenocarcinomas of known primary site, metastatic to omentum or lymph nodes were used. Immunostaining for HAM56 was performed on formalin-fixed, paraffin-embedded tissue after antigen retrieval with microwave pretreatment. Positive immunostaining was shown as membrane or cytoplasmic staining with luminal accentuation. Nonspecific staining in necrotic debris or mucin was excluded. Immunoreactivity for HAM56 was found in 37 metastatic adenocarcinomas; 22 of 31 cases (71%) of ovarian origin, 7 of 33 (21%) of colonic origin, 4 of 16 (25%) of gastric origin, 3 of 6 (50%) of biliary origin; and 1 of uterine origin (100%). Negative staining was found in adenocarcinomas from the pancreas (n = 2), cervix (n = 2), and fallopian tube (n = 1). These findings suggest that HAM56 reacts with adenocarcinomas arising from several origins though with a higher frequency in ovarian tumors. It is thus not specific for ovarian carcinomas and is, therefore, not a useful tool to help distinguish adenocarcinomas of unknown origin.