Chromatography of whole sera revealed that the areas of Rh antibody activity were generally continuous and wide. The incomplete antibody activity appeared in the early part of the chromatogram; the complete, in the latter part. The serum containing the mixed type of complete and incomplete antibodies showed activity in both regions (Fig. 1). In all cases the activity against ** f cells was spread over a wider area than that with ** f cells, regardless of the type of test (saline, albumin, indirect Coombs) used for comparison. The insoluble material resulting from dialysis against starting buffer always showed strong activity. In fact agglutination of ** f cells in saline could be produced by the insoluble material from sera containing ``only'' incomplete antibody activity. This was later known to be the result of concentrating the minute amount of complete antibody found in these sera; when the insoluble fraction was suspended in a volume of saline equal to that of the original serum sample, no complete antibody activity could be detected.