The bactericidal activity of fresh raw milk from a number of cows has been tested with the non-hemolytic mastitis streptococcus. By using this organism and other means we were able to rule out the action of agglutinin. The milk of all cows examined inhibited the growth of the streptococcus for definite periods. The length of the inhibition period varied; the milk from some cows prevented growth for 8 hours, that of others for only 4 or 6 hours. The inhibitory action may be as strong in the milk of a young cow in its first lactation period as in that of an old cow known to be resistant to udder infection. It is possible to absorb the streptococcus inhibitory substance by first inoculating the milk with B. bovisepticus. We were unable to show that the substance was increased by artificial immunization of cows with the streptococcus.

Whey obtained by the action of sterile rennet solution inhibited the growth of the streptococcus to about the same extent as the milk from which it was obtained.

We infer that the substance originates in the udder since it differs from blood alexin in its resistance to heat, it is not increased in the whey although the blood proteins are more concentrated, and it is not increased in the milk when the cows are artificially immunized or repeatedly exposed to natural infection.

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