From October 1, 1979 to September 30, 1981 a prospective study on the side-effects of BCG inoculation was conducted in the cities of Duisburg, Essen, and Mülheim, in very close cooperation with practising pediatricians, public health authorities, and obstetric departments. Particular consideration was given to suppurating regional lymphadenitis. In the period covered by the study 21.294 infants from 23.390 live births (i.e., 89%) were BCG-vaccinated using the Copenhagen 1331 strain manufactured by Behringwerke. A suppurating regional lymphadenitis occurred in 66 cases, i.e., a complication rate of 0.3%. The conversion ratio was high. Out of 8.144 documentation forms that could be assessed 6.840 (84%) attested to a positive tuberculin reaction caused by prophylactic test U 4. If the result of this test was negative a second test, after Mendel-Mantoux, strength 10, was used. According to this a total of 7.311, i.e., altogether 98.4% of the inoculation results evaluated, were positive. Both the conversion ratio as well as the incidence of ulcers at the vaccination site depend to a slight degree on the concentration of units in the vaccine which are capable of reproduction, to the extent that at higher concentrations the conversion ratio increases, as does the number of ulcers at the vaccination site.