The presence and distribution of interleukin-2 (IL-2) and its receptor complex (Ralpha, Rbeta, Rgamma) were studied in 52 women who were clinically and histopathologically diagnosed with breast tumours (17 in situ and 35 infiltrating), and in 13 women with benign fibrocystic lesions in the breast.Immunohistochemistry with antibodies against IL-2, IL-2Ralpha, IL-2Rbeta and IL-2Rgamma was used. A comparative semiquantitative immunohistochemical study between the three breast groups (fibrocystic lesions, in situ tumours and infiltrating tumours) was performed.IL-2 and its three receptor chains were immunodetected in the cytoplasm of epithelial cells. The three receptor chains were also detected on the cell surface. In fibrocystic lesions, immunoreactions to IL-2 (38.5% of cases), IL-2Ralpha (53.8%) and IL-2Rbeta (30.8%) were very weak, whereas immunoreaction to IL-2Rgamma (46.1%) was somewhat more intense. In in situ tumours, the percentages of cases that immunostained positively for IL-2 and its three receptor chains were similar to those observed in fibrocystic lesions, but immunostainings of the four antibodies were more intense. In infiltrative tumours, the percentages of positively stained cases and also immunostaining intensities were approximately twice that found for in situ tumours. Within infiltrating tumours, the percentage of cases showing immunoreaction to IL-2 and their three receptor chains was higher in the patients with lymph node infiltration at the time of surgery.The development of breast tumour is associated with an increased expression of IL-2 and its three receptor chains, and this expression also seems to be associated with the malignancy of the tumour.