An enzyme-linked immunosorbent assay (ELISA) was developed for detecting antibodies in anaplasmosis with an antigen prepared from infected bovine erythrocytes. Protein A labeled with horseradish peroxidase was used as conjugate. A comparison was made of results of ELISA, complement fixation test (CFT), and card test (CT) on sera from 97 cows in a herd in which anaplasmosis had been diagnosed. Positive ELISA reactions were observed in serum dilutions of 1:20 or greater in each of 26 cows positive by CFT and in 18 of 22 cows suspected by CFT. Of 49 cows negative by CFT, 31 were negative by ELISA. Positive ELISA reactions were observed in 45 cows positive by CT; 27 of 44 cows negative by ct were negative by ELISA. No ELISA reactions were detected in the sera of 23 cows found negative in the CFT and CT or in 8 cows negative in CFT and positive in CT. No positive CFT, CT, or ELISA reactions were observed in sera of cattle in a noninfected herd.