Summary: The comparative rates of dialysis of crystalline ovalbumin and of the allergenic protein-carbohydrate fraction, CS-1A, from cottonseed were determined. CS-1A was detectable, by gross anaphylaxis, in the dialysates from mixtures of ovalbumin and CS-1A, and from mixtures of ovalbumin and cottonseed extracts, or from a solution of CS-1A alone, 30 minutes after the beginning of dialysis. Ovalbumin could not be detected in dialysates from these mixtures or from a solution of ovalbumin alone until dialysis had proceeded for a week or more. CS-1A was, therefore, shown to contain a readily diffusible antigen. The predominant antigen appearing in the dialysate from unfractionated aqueous extract of cottonseed is closely similar to or identical with the allergenic proteic fraction, CS-51R.