The presence of the microorganism, cortical hyperplasia and germinal centers was detected in the thymus of rats infected with 10(7) viable Cryptococcus neoformans cells and immunized at 7 days afterwards with 2.5 mg (0.1 ml) of human serum albumin (HSA) incorporated to complete Freund's adjuvant (CFA) (Group 2). There was no modification of the glandular structure in the thymus of the animals only immunized with HSA-CFA (Group 1). The weight of the thymus of group 2, animals infected and immunized, was increased compared with the weight of the thymus of group 1 animals, this became evident by the increase of the thymic index (TI) (p less than 0.005). This rate was obtained calculating the thymus weight/total body weight ratio x 1000. Thymic cells (10(7) cells in 1 ml) obtained from both groups of animals were transferred to normal syngeneic rats of the same sex. The recipient rats were immunized with HSA-CFA 24 h later and 14 days after the transference, the response of delayed-type hypersensitivity (DTH) was studied in them. It could be observed that the thymic cells coming from group 2 animals were capable of suppressing significatively the afferent pathway of the DTH response to HSA when compared with the response of the animals that received cells coming from group 1 rats (p less than 0.0001).