A notion of the dynamic morphotype was developed as a conjunction between cell shape and migration. This enabled the investigation of the relationship between malignancy and patterns of dynamic morphology in neoplastic cells in vitro. Time-lapse cinemicroscopy was used to analyse the cell behaviour of three rat neoplastic cell lines (K2, T15, and A8), differing in metastatic potential, that were instrumental in revealing a coincidence between high migratory activity and appearance of the 3D structure of actin cables in high-malignant A8 cells (Pokorná et al., 1994). A set of criteria was established for visual classification of cell morphology. Matching the pattern of cell morphology with locomotory activity led to identification of four dynamic morphotypes. Cell speed was determined by tracking and the dynamic morphotypes assigned by the operator. All the three cell populations were studied for incidence of the dynamic morphotypes in culture media differing in pH: 6.6 simulating acid extracellular condition in tumours, physiological 7.4, and alkaline 8.2. The results showed that acid pH stimulated motile activity in the intermediate-malignant T15 and most malignant A8 cells. The T15 and A8 cells also manifested a prolonged continuation of fast locomotion in the early G1 phase and displayed a prevalence of two fast moving dynamic morphotypes: asymmetric stellate and triangle with leading lamella.