From a rabbit lymphoid tissue, twice immunized with a Salmonella ch. suis vaccine, was obtained a dialysable leucocyte extract (DLE) (m. w. 10,000Da; protein content 1.14 mg/ml; content of ribose 2.7 mg/ml; A260/A280 ratio 2.17 and pH 6.8). By gel filtration on Sephadex G-25, six peaks were obtained and activity was found in peak IV. The activity of the extract was determined by a dermo-application test (DAT) on 10 cows. The protective effect was tested by a challenge with Salmonella ch. suis and Salmonella dublin pathogen strains on white mice intraperitoneally treated with DLE. The DAT proved to be positive in 8 of the 10 cows. When applied on white mice, it induced a high specific protective effect against Salmonella ch. suis (70%), but not against Salmonella dublin infection.

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