It has been demonstrated in the preceding report (Bach, M. A., Beckmann, E. and Levitt, D., Eur. J. Immunol. 1984. 14: 589) that phosphorylcholine (PC) on the bacterium Streptococcus pneumoniae R36a stimulated polyclonal as well as anti-PC plaque-forming cells (PFC) in mouse spleen in vivo. In this study, red blood cells from BALB/c mice (MRBC) were either conjugated with PC, 2,4,6-trinitrophenyl (TNP) or treated with phospholipase A2 (PLA2) to expose PC on the cell membrane (determined by hemagglutination with the anti-PC myeloma HOPC8). When BALB/c mice were immunized i.v. with the conjugated or enzyme-treated MRBC, a significant polyclonal antibody response occurred (p less than 0.05) using PC-MRBC or PLA2-treated MRBC, but not with TNP-MRBC or sham-treated MRBC. No anti-PC or anti-MRBC immunoglobulin-secreting cells developed after immunization. Repeated immunization with PC-MRBC resulted in similar levels of protein A PFC after each immunization but no anti-PC, anti-MRBC or anti-PC-MRBC PFC. Thus, PC on R36a or isologous RBC stimulated increased numbers of splenic plaque-forming cells. In the case of R36a, 10-25% of these PFC produced antibodies directed towards PC. In contrast, PC-MRBC or PLA2-treated MRBC, failed to evoke any anti-PC antibody responses.