Historically, red blood cells (RBCs) with partial D antigens have been defined serologically by their pattern of reactivity with polyclonal and monoclonal anti-D. Although numerous variants have been described in tests with well-characterized monoclonal anti-D, definition remains difficult to ascertain serologically. RBCs of known partial D type were tested with LOR-15C9 (a monoclonal anti-D) and commercial anti-D by the tube indirect antiglobulin test (IAT), by micro typing system IgG gel cards, and by immunoblotting. By IAT, LOR-15C9 reacted strongly with DIIIa, DIIIc, DVa, DVI, DVII, and DFR RBCs in addition to RBCs with common D antigens; weakly with DII, DNU, and DIIIb RBCs; and not at all with DIVa, DIVb, DBT, or R0 Har RBCs. Reactivity was variable (1+ to 4+), with RBCs classified as weak D (Du). As expected, the commercial anti-D agglutinated all D variants and weak D RBC samples by the IAT and by using IgG gel cards; however, the reactivity with DVI RBCs was weaker than with LOR- 15C9. By immunoblotting, LOR-15C9 detected a band with an apparent molecular mass of approximate Mr 30,000-34,000 in membranes prepared from D-positive, DIIIa, DIIIc, DVa, DVI, DVII, and DFR RBCs and an additional band of Mr 20,000-22,000 in membranes prepared from DVI RBCs. No band(s) was detected in membranes from DII, DNU, DIIIb, DIVa, DIVb, DBT, R0 Har, weak D, or D-negative samples. LOR-15C9 provides a useful tool to identify positively DVI samples and thereby differentiate this partial D from other D variants and from weak D samples.