Heterologous antisera were raised by inoculation of rabbits with fairly pure suspensions of trophoblast, lymphoid, fetal (excluding placental components), epidermal, decidual, and renal cells from Fischer rats. After absorption of hemagglutinins, these antisera were assayed for abortifacient activity by intramuscular inoculation into time-mated Fischer females.

The anti-trophoblast serum aborted all the recipients, but had some nonspecific activity in that it caused the deaths of 12% of them. Anti-lymphocyte serum was more toxic and less potent as an abortifacient. None of the other sera harmed either the mothers or their fetuses. However, the anti-kidney cell serum caused kidney lesions.

Absorption of the anti-trophoblast serum with lymphoid cells completely removed its toxicity for the pregnant females without perceptible impairment of its abortifacient activity. However, the latter could be removed by absorption with trophoblast cells.

(a) The lymphocyte-absorbed anti-trophoblast serum was equally effective in aborting rats over the range 7–18 days postconception; (b) a single injection of 1 ml was sufficient in most cases, and (c) it was equally efficacious in terminating gestation in rats irrespective of their genetic constitution, and (d) its effect was highly species-specific.

These findings support the premise that a unique antigen (or antigens), of possible clinical significance, is associated with trophoblast cells.

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