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The idea of combining more than one vaccine into a single product to be administered by injection or by other means is far from new, as the first attempts were made over 70 years ago. The obvious advantage of combining vaccines against diphtheria (D), tetanus (T), pertussis (P), Haemophilus influenzae type b (Hib), poliomyelitis and hepatitis B (HB) into a single product for use in children has promoted the further study in recent years of both pharmaceutical and immunological interactions between vaccines. On the pharmaceutical side, factors such as pH and ionic strength of the medium, the presence and type of adjuvant, and the type of preservative used influence the immunogenicity of the different components making up a combination vaccine. The rules of the game are only partly known: consider that while adsorption of D and T toxoids to aluminum compounds is needed in order to enhance their immunogenicity (1), conversely adsorption of Hib polysaccharide (PRP) conjugates to aluminum salts decreases Hib immunogenicity (2). Another example is the decreased immunogenicity of inactivated poliovirus vaccine (IPV) in the presence of thiomersal (also known as thimerosal), which is used as a preservative in diphtheria-tetanus-whole-cell-pertussis combination vaccines (DTwP) (3).