The growth and productivity of an Sp2/0 cell line, F3b10, expressing a recombinant antibody (rAb) and BHK21 cells expressing either the same rAb from the same plasmids (BHK.IgG) or secreted alkaline phosphatase (SEAP) (BHK.SEAP) were investigated. The F3b10 line was grown as a single cell suspension. The BHK lines were grown either as suspended natural aggregates or on Cytodex 3 microcarriers. The data for F3b10 showed that the cell-specific rAb production rate (Qs(rAb)) increased in parallel with increases in the specific growth rate (mu). A similar result was obtained for suspended aggregate cultures of both recombinant BHK cell lines. In contrast, for microcarrier cultures of both BHK cell lines, Qs(product) increased as mu decreased. This report shows that the relationship between cell growth and Qs(product) for the cell lines and products studied is dependent upon the culture process. In systems where recombinant cells are growing as a single cell suspension or within a natural suspension aggregrate, Qs(product) increased with increases in mu. In such systems, the cells have a rounded morphology. When cells were grown on microcarriers, Qs(product) decreased as mu increased. Cells growing attached to a surface are flat and elongated. The observed differences in the relationship of Qs(product) to mu are correlated with changes in cell morphology. The relationship between Qs(product) and mu is also affected by the choice of cell line.