Some anchorage-dependent animal cells can form natural aggregates in stirred tanks. Baby hamster kidney (BHK) natural aggregates are described and characterized. Total cell concentration and viability could be obtained after aggregate mechanical dissociation, with negligible cell lysis and no change in cell membrane permeability. During a normal batch run, aggregates were formed immediately after inoculation, a few spherical aggregates increasing in size during the initial growth phase. At the end of the growth phase, an increase in aggregate concentration was observed, without a considerable increase in aggregate diameter. At the end of the batch run, 160 h after inoculation, aggregates disintegrated into smaller, non-spherical units, following, a sharp viability decrease. Cell concentrations of 1.2.10(6) cells/ml were obtained, with 60% of the total cells being in aggregates; the cell concentration in aggregates achieved 5.10(8) cells/ml, with a porosity of 55%. Viability was consistently in the range 85-90%, both for aggregate and suspended cells.