The majority of adult B lymphocytes in the mouse bear two immunoglobulin isotypes, IgM and IgD (μ(+)δ(+) cells) (1). A small population of IgM-bearing cells lacks, or expresses very low levels of IgD (μ- predominant [μp] cells) (1). These cells are believed to constitute a less mature subset of B cells analogous to neonatal B cells (2). Based on the time during ontogeny when responses to T-independent (TI) and T-dependent (TD) antigens appear (3, 4) and the ability to block in vitro responses with anti- μ or anti-δ (5, 6, D. Mosier, personal communication), it has been suggested that the precursors of two TI-1 responses, trinitrophenyl (TNP)- Brucella (TNP-BA) and TNP-lipopolysaccharide (TNP-LPS) are μp cells (5, 6), whereas the precursor for a TD response, TNP-sheep erythrocytes (TNP-SRBC), bears both IgM and IgD (6). However, the possibility cannot be excluded that IgD is present on some or all of the TI precursors, but that it is not obligatory for triggering. In the present experiments we have examined the phenotypes of TI and TD precursors by treating cells with C' and either anti-μ or anti-δ before stimulation with antigen. Our results suggest that the majority of B cells that respond to TNP-BA, TNP-LPS, and TNP-SRBC bear IgD, even though in the case of the two TI antigens, IgD is not required for triggering.