The phenomena that many mammalian cells won't culture properly unless they're grown in single layers of cells anchored to glass or plastic substrates. This makes it difficult to culture such cells on a large scale. Only transformed cells, hybridomas and hematopoietic cells are easily grown in suspension cultures, which are preferred because they're easier to maintain.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further
information see the BioTech homenode.