We have previously observed that some functional characteristics of pe
ritoneal macrophages (MOp) are altered during syngeneic murine pregnan
cy. To determine if these alterations are related to the immunological
stimulation that the embryo produces on the mother. we evaluated MOp
activity in allogeneic pregnancy. We also compared expression of the l
a antigen, ability to phagocyte and reduce nitro blue tetrazolium (NBT
) and to produce interleukin-1(IL-l) in allogeneic and syngeneic pregn
ancies. We observed that at Day 7 of pregnancy the increment in MOpIa percentages was more evident in allogeneic (P<0.05) than syngeneic pr
egnancies, and that these values remained high during the second week
of gestation. We also observed a significant decrease in the macrophag
es that reduced NAT during the first week both in allogeneic and synge
neic pregnancies. Yet, in the former, the percentages of MOpNBT+ were
still low in the last week of pregnancy (P<0.05). No differences were
found in IL-1 production or in estradiol and progesterone levels betwe
en the 2 types of pregnancies. Thus, it is possible to postulate that
during the first week of pregnancy the strong antigenic challenge that
the embryo represents may activate MOp and that this activation could
be augmented when major antigenic differences between mother and embr
yo are present.