In 1987 the definition of an Rh-negative donor in the Netherlands was changed from C-, E- as well as Du-negative to D-negative only. The use of 2 different strong anti-D sera without an antiglobulin phase (Du test) was considered sufficient to reveal the clinically important D antigen. In applying this policy, we identified 32 donors in 13,500 consecutive blood donations whose indirect antiglobulin test (IAT) (Du)-positive red cells gave negative reactions with at least 2 out of 11 anti-D sera and whose cells might therefore be typed as Rh(-D-)-negative in routine investigations. IgG anti-D used by a one-stage bromelain technique and anti-D with modified IgG appeared to be relatively insensitive in detecting Du in this study. Polyclonal anti-D in an enhancement medium and monoclonal anti-D scored better, although differences exist between the products of various manufacturers. It is suggested that if IAT (Du) testing is omitted, only anti-D sera with a high index of detectability of cells expressing weak D antigens should be accepted.