“…Sensitivities of indirect haemagglutination techniques are reported to be approxima tely 0.005 fig Ab N/ml [10] while the tannic acid method permitted the detection of 0.003-0.006 fig Ab N [11,12]. Pool C (0-60), a purified extract obtained from small ragweed pollen [13,14] which was used in the present study to coat the tanned red cells, has been shown to be an effective agent for this purpose [15,16,17]. Other ragweed extracts, which differed in heterogeneity and allergenic activity, were similarly used for haemagglutination, but in each case the haemagglutination titer of a given sample of RPAb was diminished relative to Pool C (0-60).…”