Mice and dogs, were treated iv with the cytostatic proteins abrin and ricin and observed for clinical, biochemical, and morphological aberrations. In both mice and dogs death occurred within a narrow dose range. Dogs given toxic doses of ricin and abrin showed weakness, anorexia, apathy, and moderate fever. No signs attributable to the central nervous system were observed. Dogs dying from intoxication expired after 15-40 h. After nonlethal doses the animals recovered, apparently completely, in 1-3 wk. No delayed changed were observed in dogs after 4 mo. Abrin and ricin, in contrast to most other cytostatic agents, did not inhibit myelopolesis. However, after sublethal dpses a rapid but translent decrease of peripheral thrombocytes was observed. No evidence for specific liver damage or impairment of kidney function was obtained. Few abnormalities were observed at autopsy or on microscopic and electron microscopic examination of the tissues, in contrast to the findings of some earlier investigators. The results indicate that in mice and dogs given sublethal doses of highly purified toxins the symptoms are reversible. There was no finding militating against a phase 1 clinical trial.