Malignant invasion in co-cultures of spheroids from the glioma cell line GaMg into brain cell aggregates (BA) was determined by two different techniques: by confocal laser scanning microscopy (CLSM) and by conventional light microscopic observations of semi-thin sections obtained from co-cultures. The remaining BA volumes were detected by CLSM in vital dye-fluorescent-stained co-cultures. The same specimens were fixed and embedded in Epon, and cut for histologic and morphometric analyses. The results show that CLSM can be used for continuous determination of progressive glioma invasion. Compared to the light microscopic observations, the degree of invasion appeared slightly lower when analyzed by CLSM. We conclude that the CLSM provides the possibility for continuous studies on interaction between normal and malignant cells. Therefore it considerably improves existing methods for studying tumor cell invasion in vitro.