Transplacental transfer of human recombinant CSF-1. Gestational day-17 pregnant mice were injected intravenously with human recombinant CSF-1 and killed at the indicated times postinjection, and human CSF-1 levels were measured by a specific RIA in (A) whole fetuses and placentas, with at least five of each measured at each time point in this one of several representative experiments, and (B) fetal sera pooled from individual litters at each time point and the corresponding maternal serum, with the data representing one of several experiments. Where indicated, placental tissue CSF-1 concentrations differed from those measured in fetal tissues at #P = .06, $P< .01, and *P < .001, respectively. Multiple comparisons over time showed differences, where indicated, of^aP < .025, ^bP < .005, and^cP < .001, respectively, versus 1 minute and^dP < .001 versus 10 minutes.