In Indonesia responsibility for immunizations is placed on local government health centres and on the nurses who provide the immunizations at each centre. An on-the-job peer training programme for these nurses, which was designed to improve the immunization performance of poorly performing health centres in terms of coverage and practice in Maluku province, was evaluated. Experienced immunization nurses were sent to health centres where nurses were inexperienced or performing poorly; the experienced nurses spent 1-2 weeks providing on-the-job training for the less experienced ones. An evaluation of the 13 centres that participated in the programme and the 95 that did not found that the programme increased both immunization coverage and the quality of practice. Coverage of diphtheria/pertussis/tetanus (DPT), polio, and measles vaccinations rose by about 39% in all 13 participating centres when compared with non-participating centres, and by about 54% in the 11 centres that had a functioning transportation system during the year after training. These results reflect increases in the actual number of doses given and improvements in the accuracy of reports. Potential threats to the study's validity were examined and found not to be significant. The out-of-pocket cost of the training programme was about US$ 53 per trainee or about US$ 0.05 per additional vaccine reported to have been given. The marginal cost per additional fully immunized child was estimated to be US$ 0.50.