Toxoplasmosis was thought to be a minor problem on a lowland sheep farm purchasing some 200 ewe lambs annually. Serological investigations and production records suggested a more substantial problem with most of the ewe lambs seroconverting during their first pregnancy, many while still at grass and receiving no supplementary food. Only 60 live lambs were produced per 100 ewe lambs tupped. It was known that cats occasionally inhabited the sheep houses, from which straw bedding was spread onto the pastures after lambing. Trials over two years with ewe lambs grazing either 'clean' or 'dirty' pastures indicated that the straw bedding was the most likely source of infection. Serial serological studies not only defined the time of infection but also showed that the ewe lambs could be kept free of infection and produce more live lambs; 58 live births came from 81 'clean' ewe lambs compared with 21 from 50 'dirty' ewe lambs.