Continuing additions of new vaccines to routine infant vaccination schedules have prompted concerns about potential interactions between vaccine components reducing desired protective effects. One hypothesis is that increasing loads of carrier protein may interfere with immune responses to polysaccharide components of co-administered glycoconjugate vaccines. Based upon a critical appraisal of existing evidence, however, neither carrier protein type nor dose adequately explains observed interference. Moreover, in five clinical trials, enhancement of anti-polyribosylribitol phosphate Haemophilus influenzae type b antibody has been demonstrated after co-administration of monovalent meningococcal C conjugate vaccine with tetanus toxoid carrier. Empirical observations do not fit well with carrier-induced epitope suppression as an underlying mechanism of interference. Thus, co-administration of conjugate vaccines can have positive as well as negative effects, and predictors of vaccine interactions are still lacking.