The availability to qualified laboratories of the stable cell strain in continuous culture satisfies the need by physicians and epidemiologists for a practical laboratory test of viral infection. With aid of this test the diagnosis of illness suspected on clinical grounds to be mild, atypical, or abortive poliomyelitis, aseptic meningitis, or encephalomyelitis, may be completed. The morphologically stable human cancer cell strain, HeLa, reveals the activity of a wide range of viruses by total destruction after infection. Conversely, antibody in the patient's serum is detected by its capacity to block the cytopathic action of homotypic virus. This inexpensive, rapid and accurate diagnostic test can and has been used in many laboratories, particularly for isolation and immunologic typing of polio virus. Throat washings, stool samples, and blood or tissue specimens from patients suspected to have poliomyelitis or other disease of the central nervous system provide material for study. Commonly, results can be obtained in from 1 to 4 days. HeLa cells have been used at the University of Minnesota for study of more than 3500 patients. Wild strains of poliomyelitis virus were recovered from more than 900 patients, and identified by immunotype. The methods described here may be used or adapted by qualified workers to epidemiological studies or other investigations of the most intimate host-parasite relationships at the cellular level.