Cross-reactivities of monoclonal reagents for red blood cell (RBC) typing are seen very rarely, e.g. in some reagents for testing of ABO or Lewis antigens. We report on two patients in whom we observed weak reactions using a monoclonal anti-c reagent containing clone MS35, but no reactions with anti-c reagents containing other monoclonal or polyclonal antibodies. To resolve this discrepancy, we studied RHCE exon 1 (Ex1) and 2 from genomic DNA of one patient and performed quality controls of the false-positive reacting anti-c reagent. RHCE Ex1 showed no abnormalities as well as RH Ex2 examined with primers specific for Ex2 D/C. No amplification product of RH Ex2 was received with primers specific for c. A titration study with untreated, papain-treated and sialidase-treated adult and newborn RBCs showed anti-i reactivity of the false-positive reacting reagent. This reactivity could not be removed by absorption with c-negative newborn RBCs without reducing the anti-c titre in the same way, indicating a cross-reactivity. Some manufacturers give a remark on a possible cold agglutinin activity of their monoclonal anti-c reagents in the instruction leaflet, but in order to avoid such irritation, we recommend to remove this cross-reactivity by dilution during the manufacturing process.