Myxomatosis is a major viral disease of the European rabbit (Oryctolagus cuniculus). Two forms of the disease (nodular and amyxomatous) exist. The clinical diagnosis of the nodular form is easily performed on the basis of typical skin lesions whereas that of amyxomatous forms must be based on virus isolation or detection of specific antibodies to myxoma virus (MV). The seroprevalence of MV was studied between March 1998 and February 1999 in 16 farms from three European countries considered free of myxomatosis on the basis of the absence of typical clinical signs. MV antibodies were detected by enzyme-linked immunosorbent assay (ELISA) (sensitivity 100%, specificity 90%) in all 16 farms; the seroprevalences corrected for test inaccuracy (95% confidence interval) were 55+/-7.7% and 37+/-6.1% for does and broilers, respectively. The association between herd sizes, types of rabbitries, and presence of recurrent respiratory or digestive troubles and seroprevalence of MV antibodies was tested in logistic multiple regressions. In all models, the seroprevalence of MV antibodies was significantly higher in herds (does and broilers) with recurrent respiratory or digestive troubles than in herds without these problems. The seroprevalence was also higher in herds (does and broilers) where animals were housed totally or partially in outdoors rabbitries than in totally enclosed rabbitries. The effect of herd sizes on the presence of MV antibodies was the same in does and broilers; intermediate sizes were at lower risk than the smaller and larger ones.