The immunodeficient (nude) mice were chosen as a model to verify the in vivo stimulating activity of bacterial antigens on the humoral immune response. By using an immunoperoxidase technique, the Ig+ cell content in the gastro-intestinal mucosa of mice was evaluated after oral treatment with a mixture of bacterial antigen fractions (trade name Colopten). Treatment for 15 days was able to induce a significant increase in the proportions of Ig+ cells in both the jejunum and ileum. In contrast, the number of Ig+ cells was significantly increased after 30 days of treatment throughout the gastro-intestinal tract. Based on the staining intensity, a semiquantitative evaluation of the Ig content of the cells was made. Strongly stained Ig+ cells were localized into the gastro-intestinal mucosa during treatment and appeared to be the prominent lymphoid cell population in the small bowel after prolonged administration of Colopten. The morphological analysis of tissues showed that after treatment Ig+ cells tended to be collected within the mucosa rather than being isolated as in untreated animals. Therefore, these results demonstrate that oral administration of Colopten was able to elicit a local humoral immune response in an animal model for severe immunodeficiency.