Titration studies with IgG anti-A antibodies revealed similar activity at both 4 and 37 °C. The saline activity of IgG anti-A antibodies generally increased as the papain titre increased with both cord and adult A, and A2 cells. Sera with papain titres >256 had saline titres in the range 32-256 at 37 °C. Estimations of the IgG anti-A content of ‘immune’ anti-A sera made by titration following partial neutralization revealed a decreasing quantitation of the results with increasing saline IgG anti-A titre, as shown by comparison to the 2-ME method.