A murine hybridoma cell line (167.4G5.3) was adapted to grow in different serum concentrations over a six month period of time. Adaptation to low serum and to serum-free media improved growth rates, but at low serum (1.25%) the antibody productivity was diminished. Flow cytometric analysis showed the presence of two distinct cell populations with respect to intracellular and surface antibody concentrations. The loss in antibody productivity during adaptation could be attributed to the appearance of a low antibody-containing cell population. Cultures maintained at high serum concentrations did not loose the original high antibody productivity. In a separate experiment the kinetics of growth improvement and loss of antibody production were studied for adaptation from 5% to 1.25% serum-containing media. Over a time period of about four months, the population shifted completely from high-producing cells to low-producing ones in response to the 1.25% environment. A shift-up from 1.25% to 20% serum resulted in the elimination of the low producing population. These results suggest that, for the cell line used, serum-containing factors prevent the loss of antibody productivity.