Toxoplasmosis is a common infection in man and animals worldwide but is predominantly asymptomatic. It is economically important in animal husbandry, causing abortion and neonatal deaths in sheep, pigs and goats. Toxoplasmosis during pregnancy and in immunocompromised subjects is of major public health importance, causing significant morbidity and mortality. Infection in humans is a zoonosis and is preventable. Extremely sensitive and specific techniques for the detection of the organism, such as the recently described polymerase chain reaction, should be helpful in documenting, more accurately, the prevalence of the parasite in nature, in elucidating the relative importance of the various routes of transmission to man, and in assessing the effectiveness of methods aimed at preventing the acquisition of human infection.