The leukocyte adherence inhibition test was used to monitor tumor-specific cell-mediated immunity in 15 patients who had a variety of malignant tumors and were undergoing chemotherapy alone or in combination with immunotherapy by Corynebacterium parvum. A rapid and prolonged loss of cell-mediated immunity in blood leukocytes was observed after treatment in all but one of the patients studied. Abolition of reactivity was due to the lack of production of the soluble lymphokine-like factor affecting leukocyte adherence to glass. A new phenomenon of adherence stimulation by antigen, also mediated by a soluble factor, was observed after treatment in some patients. A drop in titer or total abrogation of serum blocking factors occurred in six of six patients tested following chemotherapy or immunotherapy. The lowered levels of blocking activity persisted during treatment and, with the possible exception of one patient, were not correlated with clinical benefit.