Intracellular organelle motion within clonal pituitary tumor cells (GH3) was observed directly with a contrast enhancement, computer-video microscope system. All particles except nuclei moved in a complex fashion. Two types of particles predominated; one large and round, the other small and elongated. We classified the movements of these particles as saltation, oscillation and slow translocation. Saltation was directional movement with velocity of the order of 1 micron/sec. Oscillation was local motion occurring within 1 micron that showed no specific direction. Its velocity was similar to that of saltation. Large particles, in particular, showed the 3rd type of movement, slow translocation. The velocity appeared to be one order slower than that of saltation. We also examined the cells with fluorescent, dark-field and electron microscopies. We concluded that the large round particles were lysosomes and the small elongated ones mitochondria. The microtubule depolymerizer, vinblastine and the microfilament depolymerizer, cytochalasin D, completely inhibited all the types of organelle movement. The mechanism and significance of these organelle movements are discussed.