In earlier experiments we showed that locally administered Corynebacterium parvum (C. parvum) stimulated the T-cell system and had a beneficial effect on the recurrence rate of surgically resected cancers of the uterine cervix. In this paper we report the use of an immunoperoxidase technique to trace C. parvum antigen in the draining lymph nodes. In a guinea pig model the popliteal lymph node was studied after the injection of 70 micrograms of C. parvum in the hind footpad. At 6 h, intact bacteria were detected in sinus histiocytes. A transient granuloma formation was apparent between days 2 and 6, originating in the subcapsular and interfollicular areas. Three antigen-positive cell types were observed in these granulomas: a) cytophagocytic macrophages which were weakly positive, the antigen being distributed in clumps; b) dendritic cells with a strong, fine-granular positivity and c) some epithelioid cells with a small positive cytoplasmic rim. The majority of epithelioid cells was negative. Antigen-positive dendritic cells were also observed just beyond the granulomas in the T-dependent paracortical area. These cells are known as interdigitating cells (IDC) and present antigen to T-cells. Ten days following C. parvum injection the lymph node follicles became positive and the antigen could be detected in the long cellular protrusions of the dendritic reticulum cells (DRC). DRC probably play a part in immunological memory by trapping antigen in the form of immune complexes.(ABSTRACT TRUNCATED AT 250 WORDS)