Hybridoma cells (S3H5/gamma 2bA2) were cultivated using RPMI 1640 media with different serum concentrations at various initial cell densities. It was found that the serum requirement for cell growth was population-dependent. At an initial cell density of 1.10(4) cells/ml, the minimum serum concentration for cell growth was 5% (v/v) and reduced to 1% at the initial cell density of 1.10(5) cells/ml. When the initial density was increased to 1.10(6) cells/ml, serum was not needed for cell growth. The maximum cell density and the integral of viable cells over cultivation time increased as serum concentration or initial cell density was increased. However, specific antibody production rate was almost constant at 0.16 pg/cell.hr irrespective of serum concentration or initial cell density.