Circulating immune complexes were measured using the polyethylene glycol (PEG) precipitation assay in 29 normal nonpregnant women 108 normal pregnant women 18 preeclamptic pregnant women and 20 normal newborn infants The results were as follows: 1. In normal pregnant women there was no significant change in levels of circulating immune complexes complexes during pregnancy Also there was no significant difference in the mean levels of circulating immune complexes between normal pregnant and nonpregnant women 2. Taking into account a upper level (mean + 2. S.D.) in the nonpregnant state more preeclamptic pregnant women(8/18) than normal pregnant women in third trimester (7/47) showed levels beyond this point 3. The mean level of circulating immune complexes in normal newborn infants was about 1/4 of that in paried normal pregnant women