The sensitivity of eggs of Echinococcus multilocularis to environmental and controlled laboratory conditions was tested. Egg material was exposed and the infectivity was subsequently monitored by in vitro activation and by oral infection of the natural host, Microtus arvalis. To study the impact of environmental conditions in an endemic area of south-western Germany, eggs were sealed into bags of nylon mesh and exposed to the natural climate during various seasons. The maximal survival time of eggs was 240 days in an experiment performed in autumn and winter and 78 days in summer. A study of the tenacity of eggs under laboratory conditions revealed a high sensitivity to elevated temperatures and to desiccation. At 45 degrees C and 85-95% relative humidity the infectivity was lost after 3 h as well as after 4 h exposure to 43 degrees C suspended in water. Exposure to 27% relative humidity at 25 degrees C as well as exposure to 15% relative humidity at 43 degrees C resulted in a total loss of infectivity within 48 and 2 h, respectively. Temperatures of 4 degrees C and of -18 degrees C were well tolerated (478 days and 240 days survival, respectively), whereas exposure to -83 degrees C and to -196 degrees C quickly killed off the eggs (within 48 h and 20 h, respectively). Eggs of E. multilocularis were not killed off by exposure to various commercially available disinfectants applied according to the manufacturers' instructions and by exposure for 24 h to low concentrations of ethanol. Irradiation with 40 krad. from a 137Caesium source prevented the development of metacestodes but allowed seroconversion of infected rodents.