A new vaccine against pneumococcal infection is now available in the UK and has already been in routine use in the US for five years. The purpose of the trial is to determine the minimum number of does required to protect UK infants and toddlers, and to assess the compatibility of the pneumococcal vaccine with the other vaccines given in the UK childhood immunisation programme. The vaccine used in the US protects against seven strains of pnemococcus whereas the vaccine used in this trial protects against nine strains. Infants received either two doses at two and four months or three doses at two, three and four months of age, the latter comprising the routine infant schedule for other paediatric vaccines in the UK. The infants also received the usual vaccines against diphtheria , tetanus, polio, whooping cough and Hib together with meningitis C vaccine which is currently only used in the UK and a few other countries worldwide. A group of toddlers aged twelve to fifteen months also took apart and received either one or two doses of pneumococcal vaccine around the same time as their MMR vaccine. All children in the study received a booster dose some months later. The response to the vaccine is assessed by measuring levels of protective antibodies to the pneumococcal strains in blood samples taken at various times during the immunisation schedule.