Summary.Human parvovirus B19 is known to inhibit erythroid colony formation in vitro, but the precise stage of differentiation at which erythroid precursors become capable of supporting viral replication has not been accurately determined. In order to address this issue, haemopoietic cells derived from first trimester fetal liver were cultured in medium containing B19 antigen‐positive serum. Infected cells were phenotyped by combining immunohistology for cell‐type specific antigens with non‐isotopicin situhybridization for B19 nucleic acid. Strong nuclear hybridization signal was detected as early as 8 h after infection in erythroid precursors labelling with antibodies to glycophorin A. glycophorin C, CD43, CD36 and HLA‐ABC (pronormoblast or normoblast phenotype). Giant erythroid precursors labelling with the same five antibodies were a pathognomonic feature of infected cultures, but contained relatively little B19 nucleic acid. Hybridization signal was not detected in progenitor cells of more primitive erythroid phenotype or in nuclei of cells of other lineages, though B19 DNA was occasionally localized within the cytoplasm of macrophages. Double‐labelling with antibody Ki‐67 confirmed that proliferating cells were targets for B19 infection. Co‐detection of cell‐type specific antigens and viral nucleic acid is a powerful tool for investigating host cell specificity, and suggests that proliferating late erythroid precursors are the only haemopoietic cells fully permissive fo
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