Response to treatment with daily intramuscularly administered crude calf thymic extract (Suppressin) in 11 patients with Langerhan's histiocytosis (L.H.) is reported. In ten patients, T-lymphocytic subsets were studied before starting immunotherapy: OKT3 positive and OKT4 positive cells were reduced in four patients; OKT8 positive cells were reduced in two patients; three patients were normal. After immunotherapy, one patient entered complete remission, four patients had stationary disease, and six had marked clinical progression. Subsequently eight patients underwent conventional chemotherapy, and only three entered complete remission. This study has demonstrated the heterogeneity of immunological patterns in L.H. and justifies the necessity for investigations on the immunoregulatory mechanism of L.H.