We have previously shown that most rabbit splenic B cells cultured in a medium supplemented with 5% autologous serum require continuous polyclonal stimulation to maintain detectable amounts of surface Ig. In the absence of this stimulation B cells shed but do not replace their surface Ig. Here, we investigated the mechanism responsible for the loss or maintenance of surface Ig. We showed that the addition of inhibitors of mRNA and protein synthesis to the cell cultures completely abolished the Ig maintenance effect provided by the mitogen thereby suggesting that it did not act by 'freezing' the membrane Ig but rather by continuously stimulating resynthesis. Moreover, by labelling the surface Ig with 125I-labelled Fab anti-allotype antibody we showed that the maintenance of surface Ig by mitogen stimulation was due to the turnover of surface Ig. The cells shed and replaced their surface Ig with a half-life of about 2 h only when mitogen was present but shed without replacing the surface Ig in the absence of mitogen. Also, the B-cell mitogens, SM and LPS, were able to maintain surface Ig even at extremely small concentrations while the T-cell mitogens, Con A and PHA, failed to do so at any concentration, suggesting that direct stimulation of B cells was needed to maintain surface Ig. When spleen cells were cultured in 'crowded' conditions in the absence of mitogen they did not lose their surface Ig; under these conditions it appeared that a factor associated with the macroglobulin fraction is induced and acts in the same manner as a B-cell polyclonal activator to maintain the turnover of surface Ig. Such a factor may actually function in vivo since lymphocytes are in very close contact in the lymphoid organs. We concluded that rabbit B lymphocytes shed and replace their surface Ig with a half-life of about 2 h and that the replacement, but not the shedding of surface Ig, is dependent on continuous exogenous or endogenous polyclonal activation.