348 likely that early rescription of human embryos is closely related to point and chromosomal mutations, this indicator has been little used owing to difficulty in its assessment. We have collected about 30,000 early human pre-natal specimens arising from the interruption of pregnancy conducted through cervical dilation and curettage. Each specimen was accompanied by the obstetrical record. We found 421 intact empty sacs suggesting early death of the post-implantation embryos. With the purpose of exploring the role of genetic mutations on the occurrence of empty sacs, the possible correlation with a few factors working during the preconceptional stage was examined. For each case of empty sac, we selected two normal specimens as the control, not younger than the case and matched as closely as possible with the maternal age, for history of pregnancy including frequency of the past spontaneous abortion and stillbirth and for history of genital bleeding during the terminated pregnancy. Positive association of empty sacs was not verified with paternal age (mean +- S.D. for the cases; 33.2 -+ 6.71; for the controls, 33.3 +- 7.19), with parental consanguinity (prevalence among the cases, 1.13%; among the controls, 1.27%) or with drinking habits of the mother (prevalence among the cases, 19.4%; among the controls, 19.0%). However it is suggestive that prevalence of the smoking habit showed a higher value among the cases (18.6%) compared with the controls (13.3%). 22 Y. Shirasu, M. Moriya, F. Lienard and K. Kato, The Institute of Environmental Toxicology, Kodaira-shi, Tokyo (Japan) Mutagenicity studies on ethylenethiourea. I. Microbial assay Fungicidal ethylene-bis-dithiocarbamates are decomposed and metabolized into ethylenethiourea (ETU) both in plants and in animals. Recent studies revealed the carcinogenicity of ETU in rats and mice and its teratogenicity in rats. Seiler (1974) reported a weak mutagenicity in Salmonella typhimurium G46. We conducted a series of mutagenicity tests on ETU. The results of microbial testing are reported in this paper. Our test system consisted of the rec assay and reversion plate assay procedures, using Bacillus subtilis, Escherichia coli WP2 hcr, S. typhimurium TA1535, TA1536, TA1537, TA 1538 and TA100. Metabolic activation tests and host-mediated assays were also performed in rats and mice. The rec assay was completely negative even at high doses of ETU. The reversion plate assay was positive with 4 to 5-fold increase of revertants only when ETU was tested at high doses in strain TA1535. The metabolic activation test using S-9 Mix was negative with the strains used. Furthermore, negative results were obtained in the host-mediated assays using S. typhimurium G46. 23 Y. Shirasu, H. Tezuka and S. Nakamura, The Institute of Environmental Toxicology, Kodaira-shi, Tokyo (Japan)