A crystalline silica (standard quartz DQ12 with particle size less than 5 micrometers) is able to stimulate in Balb/c mice the production of antibodies of various isotypes to the T-dependent antigen trinitrophenylated keyhole limpet hemocyanin. Under the same experimental conditions silica was not able to stimulate antibody response to the T-independent antigen trinitrophenylated Ficoll. These results support the possibility that adjuvanticity of silica is correlated to its effects on one or more of the various functions carried out by macrophages on immune responses.