The effects of lysozyme (hen egg-white lysozyme) on lymphocytes harvested from mice bearing MCa mammary carcinoma were tested at the daily dose of 100 mg/kg given orally for 10 consecutive days. With this experimental paradigm that consists of a significant reduction of lung metastasis formation, lysozyme was seen to be capable of stimulating the recovery of the response to ConA of mononuclear cells, reduced by tumor growth; this effect was particularly evident with splenocytes and GALT lymphocytes. The effects of lysozyme depended: (i) on the presence of plastic adherent cells, (ii) at least in in vitro experiments, on the amount of lysozyme used, being higher at the dose of 250 mug/ml of incubation mixture, and (iii) on the time of challenge with lysozyme. These effects were shown either by measuring the [H-3]-thymidine incorporation into DNA or by application of the sulphorhodamine test for protein synthesis. After in vivo treatment, lysozyme significantly modified the histological architecture of the mucosal immunity of the gut, by causing a significant reduction of the number of the lymphatic nodules placed immediately under the layer of epithelial cells of villi. At the same time no significant modification of Peyer's patches was noted. Together these data stress the role of lysozyme in the modulation of host immunity and, in particular, point out the relevance of the mucosal immunity of the gut as first target for lysozyme activity.

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