In the post-war period the following BCG vaccines were successively in use for preventive vaccination in the Czech territory: (a) Copenhagen BCG, 1947-1950, (b) Prague BCG 725, 1951-1980, (c) Moscow BCG, 1981-1993, and (d) Behring BCG, from 1994 onwards. These BCG substrains can be now identified by modern methods of molecular genetics. Introducing the Moscow BCG brought about an elevated incidence of iatrogenic local and regional lymph node adverse reactions compared with the previous Prague BCG product and, as a new phenomenon, bone and joint involvements in children vaccinated at birth. The aim of this study was to analyze the incidence of postvaccination adverse reactions reported in the period from 1981 to 1993 as related to the Moscow BCG vaccine and, to demonstrate the effect of lower vaccination dosage on their frequency. The concentration of the Moscow BCG varied from 11 to 22.6 x 10(6) (average 16.2) CFU per 1 mg. In the period when full dose of BCG (0.05 mg per 0.1 ml) was applied to newborns 437 local and 195 regional lymph node complications were recorded, i.e. 0.08% of vaccinated were affected, demanding antituberculosis chemotherapy in 6.5% and surgical interventions in 24%. When the lowered vaccination dose (0.025 mg per 0.1 ml) was inoculated to newborns the local adverse reactions rose paradoxically affecting 0.1% of vaccinated but the regional lymph node reactions fell considerably to reach 0.01%; the demand for chemotherapy and surgery also fell down to 3.1 and 4.8% respectively. Bone and joint adverse involvements were recorded in 28 cases, i.e. in 3.7 per 10(5) of those vaccinated with the full dose.(ABSTRACT TRUNCATED AT 250 WORDS)