Semen samples from 14 patients with testicular cancer were analyzed by new flow cytometry techniques and by conventional semen analysis. Samples were obtained post-unilateral orchiectomy and prior to further treatment. With only 1 exception, light microscopic analysis of sperm count and morphology indicated a decrease of normal sperm production and incomplete or abnormal differentiation. Flow cytometry of semen aliquots showed, with the same exception as above, increased acridine orange staining of the sperm nuclear chromatin, attributed to abnormal chromatin composition and condensation. In addition, the chromatin structure of isolated sperm nuclei was abnormally sensitive to thermal stress. These changes were present up to 15 months post orchiectomy and suggest abnormal spermiogenesis in the contralateral testis of patients with testicular tumors. Of the 14 patients studied only 1 fell well within the normal range for all parameters measured; 2 other patients were in the low normal range for all parameters and 1 patient fell below normal for light microscope derived measurements but was normal by flow cytometry. The other 10 patients had abnormal measurements for all parameters. The assessment of spermiogenesis by flow cytometry studies of sperm chromatin appears to be a sensitive and valuable new parameter of fertility that cannot be determined by conventional semen analysis.