A number of experiments were carried out to study the thermal sensitivity of Mycoplasma synoviae and Mycoplasma gallisepticum as well as that of young embryos in vitro and in ovo. A full mycoplasmacidal effect was attained after heating in bouillon cultures for six hours at 45 degrees C (including cultures of Mycoplasma meleagridis), for two and a half hours at 50 degrees C, for ninety minutes at 52 degrees C and for thirty minutes at 55 degrees C. Yoder's method of heating to control these mycoplasmas in hatching eggs was found to be inadequate. When inoculated eggs were heated for ten hours at 45 degrees C, mycoplasmas could no longer be isolated; however, this had a highly adverse effect on the proportion of eggs hatched (33 and 50 percent for hen's eggs and turkey eggs respectively). Mycoplasma synoviae was slightly more sensitive to heating than Mycoplasma gallisepticum, whereas Mycoplasma meleagridis showed more resistance than Mycoplasma gallisepticum. After heating, atypical colonies constantly appeared on the primary plates and conversion of glucose was delayed in the cultures. Isolation by SPF embryos was found to be a more sensitive method than isolation using artificial culture media only. When fresh incubated eggs were heated for ten hours at 45 degrees C, for two and a half hours at 46 degrees C or for thirty minutes at 47 degrees C, fifty percent of embryos died, whereas one hundred per cent died after heating for sixty minutes at 47 degrees C. Thermal treatment of hatching eggs to eliminate Mycoplasma synoviae, Mycoplasma gallisepticum and Mycoplasma meleagridis is unsuitable for use in poultry practice.