SummaryThe results of a study in Lancashire and Salford on BCG vaccination by multiple puncture are reported. Six groups of children were vaccinated either with 70 or 100 mg./ml. strength of vaccine and given 6, 20 or 40 punctures. All the 898 vaccinated children tested in Lancashire and almost all (90 per cent or more) of the 866 vaccinated children in Salford in the six groups were positive to the Heaf test 10 to 13 weeks after vaccination.At 10 to 13 weeks after vaccination in Lancashire, those vaccinated with 20 needles and given the 70 mg./ml. vaccine had a significantly higher tuberculin sensitivity than those vaccinated with either 6 or 40 needles. There were no significant differences in sensitivity, according to the number of needles used, with the 100 mg./ml. vaccine in Lancashire and with the 70 mg./ml. strength in Salford. Those vaccinated with 20 needles and the 100 mg./ml. vaccine in Salford had a significantly higher conversion rate than those given 6 needles.For all methods of vaccination at 10 to 13 weeks, half of the children had two-thirds or more of the complete number of papules of induration. The majority of the minor complications reported in both areas were in those vaccinated with 20 needles.At one year after vaccination, the proportions tuberculin positive were lower than between 10 and 13 weeks in Salford, and very considerably lower in Lancashire. At one year in Lancashire, as between 10 and 13 weeks, those vaccinated with 20 needles and given the 70 mg./ml. vaccine had a significantly higher sensitivity than those given either 6 or 40 punctures; there were no significant differences according to the number of needles used with the 100 mg./ml. vaccine. In Salford at one year, the children receiving 6 punctures had a significantly lower positivity than those vaccinated with 40 needles, for both strengths of vaccine, and also than those vaccinated with 20 needles for the 100 mg./ml. vaccine.The differences in pressure required to release the needles of the guns to obtain adequate penetration may partially account for the differences between the groups in both tuberculin sensitivity and lesions produced after vaccination. The pressure required was greater for the 20-needle guns used in Lancashire than for the others. Laboratory experiments suggested several other factors affecting vaccination by multiple puncture.It appears that, in this study, 6 needles were not sufficient for satisfactory vaccination, 20 needles probably provided the most satisfactory vaccination and a 40-needle gun may be impractical. The pressure at which the needles are released must be sufficient to produce their full penetration into the skin, but further work on the multiple puncture method is needed.