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Blood samples from nine broiler breeder flocks comprising five flocks clinically affected with myeloid leukosis tumours (ML+) and four tumour-free flocks from the same commercial background (ML-) were compared for avian leukosis virus subgroup J (ALV-J) serum antibodies by enzyme-linked immunosorbent assay (ELISA), for antigenemia (group-specific antigen) by antigen-trapping ELISA and for viremia. Group-specific antigen was detected in the sera of 58.1% of ML+ birds and 46.4% of the ML- birds (P=not significant), while 45.5% of ML+ birds and 24.1% of the ML- birds had ALV-J antibodies (P=0.065). In inoculated cell culture, 64.1% of the ML+ sera were viremic compared with 16.7% of the ML- sera (P=0.001). Similar significant differences were found between the two groups of flocks when ALV-J viremia was detected by immunofluorescence using a monoclonal env antibody (P=0.004), and for proviral DNA by polymerase chain reaction using two different sets of env-gene primers, H5-H7 (P=0.001) and R5-F5 (P=0.001). Using the primer pair R5-F5 the product size was approximately 1 kbp, while some heterogeneity in size among isolates was discernable. Our results indicate that a combination of diagnostic tests should be adopted in routine examination of tumour material in order to rule out false-negative findings.