The difficulty of achieving tritium self-sufficiency in a deuterium-tritium (D-T) hybrid has been reported in relation to different hybrid designs and concepts using lithium as the breeding material. Lithium-6 enrichment has been suggested to overcome this problem, provided the contribution of /sup 7/Li to tritium production is usually negligible, as is commented on at the end of the paper. On the other hand, the use of the tritium produced in light water reactors has been proposed to feed the hybrids unable to breed their own tritium requirements. In this paper, a criterion that establishes the conditions for tritium breeding will be derived. A subcritical multiplication model will be used to characterize the blanket neutronics. The analysis points out that /sup 6/Li enrichment is not the suitable proposal to obtain tritium breeding, because it does not affect the critical parameter in this context. On the other hand, the /sup 7/Li contribution to tritium production is almost negligible in multiplicative hybrids when the natural lithium is arranged only within the blanket lattice cell (and not in extra regions in the innermost part of the blanket). For thermal and epithermal blankets, the /sup 7/Li contribution is approx. 0.2% of the total rate. Formore » fast systems, this figure goes up to 5%, even though it decreases as K increases.« less