The complement fixation reaction (CF) and indirect immunofluorescence (IF) were used to test serum from 3,000 potential blood donors at the Blood Bank of the University Hospital, Londrina, Parano State, Brazil. A total of 7.4% of positive serological reactions was the results of the two tests being concordant in 97.1% and discordant in 2.9%. The IF test was more sensitive than CF; positive results by IF with negative or doubtful results by CF were 15 times more frequent than positive results by CF with negative or doubtful results by IF. Judging by the results of the two tests, the security of selection of acceptable blood samples was greater when doubtful results or anti complementary sera were considered positive. In another group of volunteer candidates for blood donation who had donated blood on other occasions, 450 had had one to six CF tests carried out in other laboratory. Of these, 10 (2.2%) had had doubtful or positive reactions in one or more of these previous tests but had been accepted as potential donors at the time of the present study. These results demonstra te the relative nature of the process of donor selection on the basis of serological reactions. The alternative utilization of gencian violet by hemotherapy services is discussed.