The RIA and IHA methods to quantitate antipneumococcal antibodies of six capsular types (3, 6A, 8, 12F, 14, and 18C) were compared in 30 paired sera 1 month after pneumococcal immunization. Correlation coefficients for titers were significantly different from zero (p less than 0.001) for all types except 14 and ranged from 0.86 for type 3 to 0.45 for type 18C. In sera that showed a twofold or greater rise by RIA, 50% to 83% showed a fourfold or greater rise by IHA, depending on the capsular type. Conversely, in those sera showing an IHA rise of fourfold or more, 79% to 100% showed an RIA rise of twofold or more. IHA titers correlating with RIA titers greater than or equal to 200 ng AbN/ml varied from 1:16 for type 18C to 1:64 for types 3, 8, and 12F.