The present invention relates to methods and compositions for augmenting an immunogenicity of an antigen in a mammal, comprising administering said antigen together with an adjuvant composition that includes glycosylceramide, preferably .alpha.-galactosylceramide (.alpha.-GalCer). According to the present invention, the use of glycosylceramide as an adjuvant is attributed at least in part to the enhancement and/or extension of antigen-specific Th1-type responses, in particular, CD8+ T cell responses. The methods and compositions of the present invention can be useful for prophylaxis and treatment of various infectious and neoplastic diseases.