Binding of IgM antibody to normal RBCs incubated with (A) normal plasma and PBS, (B) normal plasma and urine from a subject taking DCF, (C) patient plasma and PBS, and (D) patient plasma and urine from a subject taking DCF. Mean fluorescence intensity is indicated above each histogram. Significant amounts of IgM became bound to target RBCs only in the presence of patient plasma and urine from a subject taking DCF (D). No antibody binding was detected with an IgG-specific probe (data not shown).