ABSTRACT-Fv-4 locus controls the susceptibility of mice to ecotropic murine leukemia virus (MuLV). Antiserum against Fv_4 r / s mice with BALB/c background was raised in inbred BALB/c mice. In the com-plement-dependent cytotoxicity test, the antiserum killed the hemato-poietic cells derived from Fv-4 r /- mice but not those from Fv_4 s / s mice, and the genetic cross experiments located the locus controlling the expression of the target molecule of the antibody in the close vicinity of Fv-4. In addition, the antigenic expression of the exogenously infecting MuLV suppressed the expression of the target molecule of the antiserum. The expressions of these two antigens appear to com-pete with each other on the cell surface. JNCI 1982; 68: 1005-1009. The presence of Fv-4 locus was indicated by the isolation of a mouse strain from a ddY mouse colony in Shizuoka, Japan, which was resistant to N-, B-, or NB-tropic ecotropic MuLV (1). The resistance allele (Fv-4r) was dominant ove
