A brief account is given of the aims and methods of the immunisation campaign in Cape Town. Figures show that approximately 60 per cent. of European and 50 per cent. of non-European children have now been immunised. The diphtheria incidence rate per 1,000 of the population shows a very material decrease for the first time in the 1944-45 and 1945-46 figures, which can be attributed to extensive immunisation. A comparison of diphtheria cases in the immunised with a random sample of 100 cases in the unimmunised shows a striking modification of the disease in the former. The incidence rate in Cape Town is approximately in 600 in the immunised and 1 in 50 in the unimmunised.