Abstract. Cold agglutinin levels were determined in 120 individuals ranging in age from 1 to 64 years using untreated and ficin‐treated group O erythrocytes. The geometric mean titer for untreated cells was 5.7±1.1 SEM and for ficin‐treated cells 18.7±1.1 SEM. The mean titers and scores for various blood groups were not significantly different. Titers and scores correlated significantly to total serum IgM concentration (p<0.001). Females had significantly higher cold agglutinin levels than males (p<0.001). A significant decrease of cold agglutinins occurred in the 26‐ to 64‐year age group as compared to the 11‐ to 25‐year age group (p<0.01). The data should contribute to a clearer understanding of the biological impact of cold agglutinins.