RESULTS Groups I, 2 and 3 Slides which could not be scored. Thirty-two of the samples could not be scored because of the presence in the slides of numerous cells showing a very variable intensity of staining. Deeplystaining cells which might in another context be classified as definite foetal cells were associated with many cells showing all intermediate degrees of staining (Fig. Ib). Even if a small number of cells of foetal origin were present in these films it would be difficult, if not im-possible, to recognize them. This appearance was seen in:(a) two out of the IOO samples from Group I,(b) five out of the 259 samples from Group 2, and (c) 25 out of the 398 samples from Group 3, giving incidences of 2, 1.9 and 6.3 per cent respectively. The 25 samples from pregnant women were obtained from 21 individuals, the appearance being recorded in successive samples from the same women in some instances. These samples are not included in the following results and analysis.