The benefit of the immunomodulation properties of thymic hormones in patients receiving chemotherapy for malignant diseases is yet to be defined. The efficacy of thymostimulin (TP-1) on hematological tolerance is evaluated, the performance level, the number and severity of the infections and skin reactions in two groups of homogeneous and randomized patients, diagnosed as having lymphoma and myeloma. Both patient groups (receiving or not receiving treatment) consisted of 20 persons. This observation was carried out in 96 courses of chemotherapy in each group; no significant differences having been noted in respect of the performance status at the beginning nor at the end of the study period between the 2 treatment groups. The hematological tolerance to chemotherapy, skin reactions and number and severity of infections registered were not significantly different between the 2 groups. In conclusion, we were unable to confirm an improvement in the clinical parameters of the myeloma and lymphoma patients receiving chemotherapy and thymostimulin during the period of observation.