The laboratory of the Animal Health Service of South Holland investigated 2410 cases of spontaneous abortion (twins (112) were considered one case). Seventy-eight afterbirths without foetuses were also examined. The aim of the study was not only to find the cause of abortion but also to see whether there was a relation between the results of the different bacteriological, virological and histological tests used. The presence of IgG in the blood of foetuses older than 5 months was also determined. A micro-organism was isolated from 25% of the foetuses. On the basis of results of other tests, it is probable that microbial infection played a role in a further 21% of the abortions. BHV was isolated in 2.5% of the cases. Actinomyces pyogenes was found in 6% of the foetuses and fungi in 3.5%. More infections with Actinomyces pyogenes, Listeria monocytogenes and fungi were observed when the cattle were kept indoors. It is emphasised that both foetuses as well as placental membranes should be examined, and both foetuses of twins. In stable enzooties in the Netherlands, the following infections should be taken into consideration: Brucella abortus, BHV, BVD, Chlamydia psittaci and Leptospira hardjo. The authors stress the need for a more standardised approach to research, the more so because the results could have consequences not only for monitoring activities (checks for Br. abortus) but also for liability.