Five groups of 10 rabbits each were injected intravenously 2 times at 15 day intervals with either whole horse serum or one of its cold alcohol-precipitated fractions. Suitable serological and general observations were made at appropriate intervals before and after each injection. All animals were sacrificed on the 22nd day of the experiment. A study of the antemortem and pathological findings led to the following conclusions.

1. Allergie arteritis, valvulitis, and to a lesser degree, focal pericarditis, Aschoff-like nodules, and glomerulitis can be produced by several of the cold alcohol-precipitated fractions of horse serum as well as by whole serum.

2. Most of the acute arteritis was seen in rabbits receiving fraction V (albumin). These rabbits showed the largest amounts of circulating antigen, low antibody titers, low tissue sensitivity, and slight elevation in sedimentation rate and temperature.

3. There was a high incidence of chronic arteritis in the rabbits receiving fraction III which is almost devoid of albumin, suggesting that the alpha and beta globulins in addition to albumin may produce arteritis.

4. A state most nearly resembling that of acute rheumatic fever was produced by either fractions III or IV-3,4 (alpha and beta globulins). Pancarditis (pericarditis, Aschoff-like lesions, and valvulitis) was found relatively frequently. Many of the rabbits developed a high sedimentation rate, elevated temperature, and high tissue sensitivity, but little acute arteritis was found in this group.

5. Gamma globulin (fraction II) produced little reaction either in the antemortem determinations or histopathologically.

6. Glomerulitis of an acute necrotizing type was seen in a few rabbits without particular correlation to the fraction injected.

7. The frequency of involvement of heart valves in rabbit serum disease follows a pattern very similar to that of rheumatic heart disesae.

8. Attempts to correlate antemortem observations and pathological findings either on a group basis or for individual animals failed.

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