The reactions of anti-I cold-agglutinin antibodies with the red cells from normal adults (adult-I) and adult-i donors and from newborn infants have been investigated by the use of the C'1a Fixation and Transfer Test of Borsos and Rapp. Agglutination and high concentration of serum reduce the amounts of the C'1a fixation by these antibodies. Inhibition by serum is due to an inhibitor of C'1a. A single molecule of antibody is required for the fixation of one molecule of C'1a to either adult or cord cells. There was great variability in the amount of anti-I antibody and C'1a fixed by different adult cells and by different cord cells. Although the two ranges overlapped considerably, the amount of antibody fixed by cord cells was somewhat less than that fixed by adult cells. The difference between cord cells or "low-reactivity" adult-I cells and "high-reactivity" adult-I cells is not due to the presence of different or extra antigens on the latter since cord and "low-reactivity" adult I cells are able to absorb all antibody from the antiserum which reacts with "high-reactivity" adult-I cells. The difference between adult-I and cord cells (or adult-i cells) was reduced by pretreatment of the cells with papain and was increased by increasing the temperature at which the reactions were performed. These studies suggest that the difference between adult red cells and cord (or adult i) red cells with regard to the fixation of cold agglutinin antibody is due in large part to a difference in the "affinity" of the antibody and the antigen on the cell surface rather than to a major difference in the number of antigen sites.