The detection of occult blood in gastric fluid samples may be clinically important, but many current methods are too sensitive. Gastric fluid containing various concentrations of blood were subjected to analysis by four screening methods. Orthotolidine based tests (Hematest reagent tablets and Bili-Labstix) designed for urinary and fecal blood detection when applied to gastric aspirates were highly accurate, but sensitivity was so great that normal fluid yielded positive results. Guaiac slide tests (Hemoccult and Gastroccult slide tests) were less sensitive than the orthotolidine-based tests. The Hemoccult slide test was influenced by the low pH of gastric aspirates and neutralizing the acidity of the gastric fluid by addition of NaOH improved the sensitivity of the method. The Gastroccult slide test accurately determined gastric fluid pH and hemoglobin concentration greater than 300 micrograms/dl in the acid environment of gastric fluid. The Gastroccult slide test is a simple, rapid, and convenient method for initial evaluation of patients suspected of occult blood in gastric juice.