In previous studies, BALB/c mice immunized with trinitrophenyl-specific IgA protein (M315) produced by MOPC-315 developed idiotype (Id315)-specific T cells that suppressed M315 secretion in vivo. In the present in vitro studies, we show that inhibition of M315 secretion is mediated by a theta,Lyt-1-2+ cell that expresses a surface membrane receptor for Id315. The suppressor signal is a diffusable product that acts directly on M315-secreting myeloma cells. Inhibition of M315 secretion is T cell dose-dependent, Id315-specific, reversible, and occurs without any effect on MOPC-315 growth, viability, or surface membrane expression of M315. Inhibition of M315 secretion results from a selective inhibition of M315 synthesis in the myeloma cell. These studies provide new insight into the mechanisms of direct B cell regulation by idiotype-specific T cells.