1. A description is given of internal and subsurface discolorations in unsalted and lightly salted butter.

2. Six strains of the causative organism Cladosporium herbarum are described.

3. These strains are shown to be capable of reproducing the defect under experimental conditions.

4. Four of the strains tested proved capable of slight growth at 20–24° F. (−6·6 to −4·4° C.). after 3½ months.

5. None of the strains grew at normal cold store temperatures, 15–18° F. (−9·4 to −7·7° C).

6. Salt retards the development of internal spotting. Under favourable conditions it was shown that discolorations developed in 6, 10, 16 and 25 days in butter containing 0, 5, 10 and 15 per cent, of salt in the butter serum.