PENICILLIN TOXICITY IN THE GUINEA PIG MAY BE MANIFESTED IN SEVERAL DIFFERENT WAYS, AND IT IS PROPOSED THAT THESE TOXIC EFFECTS BE CATEGORIZED INTO THREE SYNDROMES: (1) toxic syndrome, characterized by acute fatal illness; (2) hemorrhagic syndrome, characterized by delayed illness with leukopenia and thrombocytopenia, and culminating in massive visceral hemorrhages; (3) chronic syndrome, characterized by retardation of growth and alopecia, a condition somewhat resembling "runt disease." A virus having some of the properties of a parvovirus has been isolated repeatedly from animals ill or dying of penicillin-induced disease. This finding has been construed as being activation of a latent virus by this antibiotic, but the relationship, if any, of the phenomenon of viral activation to the syndromes produced by penicillin and its frequent lethal toxicity is unknown. That a strong association exists, however, has been established. Of some 60 guinea pigs which received injections of penicillin three developed tumors and four others were found to have gallstones. A virus similar or identical to the guinea pig virus also has been isolated from hamsters dying of penicillin-induced disease. It is hypothesized that the absorption of endotoxin, resulting from the well known change in intestinal flora caused by penicillin, produces a state of immunodeficiency which regularly gives rise to activation of a latent virus, and perhaps, rarely, to the development of malignant neoplasms.