Plasma from rabbits injected with Leishmania tropica showed an inhibitory effect on the growth of Leishmania tropica, enriettii and donovani in vitro. This inhibitory effect was assayed by counting the viable leptomonads under the microscope, using trypan blue as a vital stain. The number of viable leptomonads in cultures containing plasma from injected rabbits decreased steadily, while in cultures of leptomonads with normal rabbit plasma the numbers of leptomonads increased rapidly, reaching a peak on the fifth day of culture. Some effect of the plasma from injected rabbits on the leptomonads could be observed after they were incubated together for 4 hours. The growth inhibitory factor in plasma from injected rabbits is thermostable at 56 °C., its action is independent of the presence of complement, and it could not be adsorbed out by the homologous organism under our experimental conditions. The degree of inhibitory activity of the plasma factor was correlated with the complement fixation titer of the plasma against a soluble antigen of L. tropica. © 1969.
