The efficacy of ribosomal preparations as mucosal immunostimulants was examined in the peripheral blood and tonsils of 14 children, before and after 28 days of oral treatment with D-53, a preparation of ribosomes from Klebsiella pneumoniae, Streptococcus pneumoniae, Haemophilus influenzae and Streptococcus pyogenes. Tonsils from 10 untreated children were used as controls. Immunofluorescence and ELISAspot were performed to analyse variations in the numbers of immunoglobulin-containing and immunoglobulin-secreting B-cells. Both isotypic and antigenic specificities of these two types of cells were investigated. Significant differences were observed after treatment in the peripheral blood as well as between tonsils from treated and untreated children. In the peripheral blood a significant increase in immunoglobulin-secreting cells directed against antigenic specificities of D-53 was the major change. In tonsils, higher numbers of specific immunoglobulin-containing and secreting cells, and higher numbers of IgA-secreting cells were induced in treated children. These data support the efficacy of D-53 as an oral immunostimulant.