1. Studies on nineteen patients to whom foreign serum had been administered for therapeutic purposes are reported. Analysis of the results obtained by following the precipitin and precipitinogen in the circulation and comparing these factors with the time of appearance, intensity, and duration of the symptoms shows that the nineteen patients fall into three groups.
2. The first group includes eleven patients. These were good precipitin formers, they had severe serum disease, and the precipitinogen disappeared from the circulation near the time that the symptoms subsided.
3. The second group includes four patients who had little or no serum disease, in whose circulation little or no precipitin was demonstrable, and in whom the precipitinogen persisted in the circulation as long as the patients could be kept under observation—from 52 to 67 days.
4. The remaining four patients form a more or less distinctly intermediate group.
5. The results lend further support to the conception of serum disease as an antigen-antibody reaction.
6. The possibility that our results indicate a factor which may be important in the mechanism of natural immunity is discussed.