Kinin levels began rising on day 3 after infection of cattle with Babesia bovis (= B. argentina) and attained a maximum value of 98% above preinfection levels by day 7. 2. Kininogen levels began falling on day 3 and reached minimum levels of 83% below preinfection levels on day 8. 3. Changes in both kinin and kininogen levels on day 3 coincided with the detection of low levels of parasites, and with a fall in packed cell volume. 4. Plasma kininase levels rose significantly 6 to 9 days after infection. Preparations of lysed and sonicated uninfected and infected red cells contained kininase activity, the respective red cell preparations being 23.9 and 11.4 times more active per mg protein than uninfected red cell preparations. The effect of pH, and the inhibitors disodium edetate, 1,10 phenanthroline and aprotinin on normal and infected plasma and on the various red cell preparations suggested that the rise in plasma kininase levels during infection was probably at least partly due to parasite products. 5. These results are discussed in relation to previous data showing that both kallikrein activation and the onset of hypotension also occur on or about day 3.
