In the last two decades, much work has focused on factors causing growth limitations of animal cells in large scale culture systems and on strategies to overcome these limitations. It is agreed that growth is limited by a combination of environmental factors and by growth inhibitory molecules produced by the cells themselves. Identification of cell lines which may be resistant to some or all of any potentially growth limiting factors would be of considerable theoretical and industrial use. The aim of the work undertaken in Section 3.1 of this thesis was to identify and characterise growth inhibitors produced by animal cells when growing in vitro. It was found that MSV-3T3 cells produce and secrete factors which either inhibit the growth or have a cytotoxic effect on epithelial carcinoma cells in vitro. Either one factor or an aggregate of factors may be responsible for the cytotoxic activity observed. This activity was variable and was seen in some molecular weight fractions and not in others suggesting perhaps that the factors responsible may exist as:- an inhibitory molecule and stabiliser entity, a precursor and protease entity or in a TGFa/TGF|3 complement existence. The results suggest that the molecules responsible for the cytotoxic activity are unusually stable entities with high heat and pH resistance. As well as endogenous growth inhibitors, environmental factors such as agitation intensity, osmotic pressure, temperature, oxygen levels and pH may also limit the growth of animal cells in vitro. The build up of inhibitory metabolic waste products such as ammonia and lactate should also be considered. Section 3.2 of this thesis presents investigations on environmental factors which may cause inhibition of animal cell growth in scaled up in vitro systems. A decrease in apparent growth rates was noted with extremes of agitation, osmotic pressure, lactate and ammonia. Of particular interest in this work was the observation that while high agitation rates caused a decrease in the growth of CHO-K1 cells, no such growth limitation was seen in their multi-drug resistant (MDR) counterparts, CHRC5, at high agitation rates. This observed agitation resistance in the MDR cells (CHRC5) was consistently noted in a number of different experiments. An increased agitation resistance was also noted in the MDR cell lines, Hep-2A when compared to its MDR sensitive parent, Hep-2 and DLKPA (MDR) when compared to DLKP. Investigations on the plasma membrane fluidity of CHRC5 and DLKPA suggests that there is a correlation between increased agitation resistance and decreased plasma membrane fluidity in the two MDR cell lines studied. If it is considered that MDR may be conferred on cells by over-expression of membrane proteins, (for example, p-glycoprotein in CHRC5 cells), then an increased protein content in the cell membrane may lead to an increased packing density giving a more rigid structure with decreased plasma membrane fluidity and so resulting in an increased shear/agitation resistance being exhibited by the cells. In additon, lactate sensitivity was noted in the two MDR cell lines, CHRC5 and Hep-2 A when compared to their parent cells.