The accuracy of the absorbed heterophile and the ox-cell hemolysin procedures was compared by testing of 1577 sera routinely submitted to the State Laboratory of Hygiene for serodiagnosis of IM. Results of both tests were in agreement with 1369 (86.8%) sera; additional 122 (7.7%) samples yielded minor disagreements in results. Major discrepancies in results were seen with 86 sera (5.4%). On basis of clinical data peripheral blood smear findings, tests comprising our IM syndrome test battery, and additional serologic tests, we concluded that in most instances of ox-cell hemolysin positive heterophile negative findings, there was evidence to support a diagnosis of IM. Such support was lacking in a majority of cases with heterophile positive, ox-cell negative findings, and in several of these patients infection with agents other than EBV was suggested. On the basis of comparisons of sensitivity, specificity, cost and ease of performance, the ox-cell hemolysin test appears to offer several advantages over the other available IM serodiagnostic procedures for a public health laboratory performing a large number of tests.