Specific reference is applied to the functional significance of gravidity, a process being equivalent to the isogeneric transfer that takes place under anomalous immunological situation, and what is specifically discussed here is the behavior of NK-activity observable throughout gravid period, some of the distinctive facts it offers being illustrated as follows:1) NK-activity began to fall with the beginning of gravidity, decreasing further with time, until it regained subsequent to delivery as high a level as of non-pregnant subject;2) The findings that removal of adherent cells and administration of indomethacin has been incidentally shown to stimulate NK-activity under gravidity, positively suggest that loss of NK-activity may be attributable to prostagrandin produced by monocytes;3) It is commonly agreed that γIFN would normally give rise to a stimulation of NK-activity. This observation, however, was found to be not absolutely true while in gravidity, whereas NK-activity reacted against γIFN 3 weeks after delivery as if the mother were non-pregnant. In addition, removal of adherent cells could entail none of change in sensitivity of NK-activity to γIFN. Altogether, it has been shown that adherent cells would not be responsible to the reduced reactivity to γIFN under gravidity;4) In sharp contrast, against rIL-2 NK-activity, either of pregnant-or non-pregnant subject, invariably behaved positively.