SUMMARYThe ACI or BDIV rats responded with decreased or increased neutrophil levels in blood after the N‐methyl‐N‐nitrosourea (MNU) administration. The F1 hybrids had decreased neutrophil counts, and the BDIV × (BDIV × AC1)Fl backcross offspring showed two phenotypes. The sex of the rats and the neutrophil response to MNU assorted independently. The results indicated that the neutrophil response to MNU was regulated by autosomal geneNrm‐1with two alleles. TheNrm‐Idregulates the decrease and theNrm‐1iregulates the increase of neutrophils in blood after the MNU administration. The results were confirmed by the SKUMIX computer program.We found that theNrm‐1gene was linked to the agouti locus (chi‐square = 10–3,P<0.001). The map distance between two genes was 33 ± 5 cM. TheNrm‐1gene thus resides on the linka