Dr. Kassai Tibor szerk.: Parasitologia Hungarica 2. (Budapest, 1969)

vival is prolonged. 4. If the development of the immune response is inhibited by prednisolone (9), antilymphocytic serum (7), whole body irradiation, or other immunosuppressive treatment (KASSAI and SZEPES,personal communication), elimination of females is retarded or does not occur, and egg output continues. 5. Changes in sex ratio fail to occur in rats made immunologically tolerant by an early primary infection (6). The results of the experiments described in this paper are taken to indicate that the rate and extent of changes in sex ratio are in some measure dependent upon the intensity of infection as determined by the number of infecting larvae. The more vigorous immunological response Induced by more intense infections results in a more rapid decrease in the number of worms and a more pronounced alteration in sex ratio. However, the proportion of females in the residual population decreased considerably even in the most lightly infected groups, a finding that is in agreement with OGILYIE's observation (8) that about 10 Nippostrongylus females are sufficient to induce a degree of immunity in the rat host. Summary In adult rats infected with 25, 50, 100, 200, 500, 1200, and 2000 Nippostrongylus larvae the author studied egg output, elimination pattern and changes in the sex ratio of the worm population. The characteristic decrease in egg output was associated with elimination of female worms from the host and not with any decrease in their egg producing ability. As the population aged females left the host more rapidly than males, causing a change in the sex ratio.