The possibility was explored of employing the indirect hemagglutination (IHA) test for the detection and identification of Colorado tick fever (CTF) virus as well as for the detection of antibodies to this virus. For this purpose, sheep erythrocytes fixed with acrolein or glutaraldehyde were sensitized with immune gamma-globulin isolated from ascitic fluid of mice immunized with CTF virus. IHA reaction employing antibody-coated erythrocytes was found to be highly specific and simple to perform, and permitted reading of the results after 1.5 to 2 hours. Preliminary fixation of erythrocytes made it possible to obtain a stable preparation that could be stored without any appreciable loss of activity during an observation period of 3 to 4 months. The IHA test can be applied for the laboratory diagnosis of Colorado tick fever.