Immunosuppression is characteristic of patients with advanced malignant neoplasms. However, none of the simple assays for immunocompetence have been found to provide results which correlate with the patients' responses to therapy. The data reported here indicate that this may be because there are artifacts in data from earlier studies which have obscured such a correlation. A simple assay which removes previously unrecognized sources of error is shown to generate data which correlate well with patient responses. In this assay, lymphocytes collected from patients with malignant solid tumors were stimulated with mitogen in their autologous plasma. The amount of radioactive thymidine incorporated during replicative deoxyribonucleic acid (DNA) synthesis was corrected to remove sources of error ignored in the previous studies. A significant improvement in mitogen-stimulated synthesis was observed within two months for patients entering remission. For patients not responding to therapy, there was a progressive deterioration in mitogen-responsiveness.