Anti-Yta is an antibody to a high-frequency (99.8%) antigen and has been reported to have variable clinical significance. A retrospective review of monocyte monolayer assay (MMA) results on sera from 79 patients with anti-Yta was conducted to determine the reliability of this assay to predict the clinical significance of anti-Yta. Results of the MMA, other serological tests, and patient transfusion histories were analyzed. The MMA was found to be a reliable predictor of the clinical significance of anti-Yta when samples for analysis were collected at least six weeks after initial detection of the antibody. Only 18.2 percent of those examples of anti-Yta tested appeared to be clinically significant.