Staphylococcal enterotoxins, Types A, B, and C, were labeled with 125I by the chloramine-T method at approximately two levels of specific activity, 40 and 4 μCi/μg of protein. Toxins labeled with high specific activity showed extensive dissociation of 125I when stored at different temperatures, including -23 C. In contrast, toxins labeled with low specific activity did not show any significant loss of 125I when stored at -23 C for as long as 2 months. Enterotoxins, whether labeled with high or low activities, formed aggregates immediately upon labeling. Aggregate formation increased in high-activity-labeled toxins on storage at -23 C, and low-activity-labeled toxins showed no significant increase in aggregate formation, even after 2 months at -23 C. The aggregated forms of the enterotoxins were either devoid of antigenic activity in solid-phase radioimmunoassay or they possessed significantly reduced antigenic activity. Thus, a decrease in binding of 125I-labeled enterotxin to specific antibody in solid-phase radioimmunoassay results mainly from (i) loss of 125I on storage, and (ii) formation of aggregates with reduced antigenic activity.