The relation between in vitro production of HeLa cell cytotoxin by strains of Shigella and clinical symptomatology was determined for 35 travelers from the United States who developed shigellosis in Guadalajara, Mexico. There were 25 patients with Shigella sonnei, eight with Shigella flexneri, one with Shigella boydii, and one with Shigella dysenteriae. These strains were evaluated for in vitro production of cytotoxin. The amount of cytotoxin did not correlate with the number of stools passed, the severity of abdominal pain, or the presence of nausea or vomiting. However, patients with strains of Shigella that produced more cytotoxic activity were more likely to have fever (P less than .02) and occult blood in their stools (P less than .004). The cytotoxicity produced by 30 (86%) strains could not be neutralized with rabbit antiserum to purified, formaldehyde-treated Shiga toxin from S. dysenteriae type 1 strain 60 R; the cytotoxicity of five (14%) of the strains was partially neutralized. When only nonneutralizable cytotoxin was considered, the presence of fecal leukocytes (P less than .04), as well as of occult blood (P less than .002) and fever (P less than .02), correlated with the amount of cytotoxin. The amount of nonneutralizable cytotoxin produced by shigella strains was related to the clinical findings. This cytotoxic activity was infrequently attributable to "Shiga toxin".