The effect of cytochalasin B (CB), nocodazole, and irradiation on the adherence and internalization of 3H-labeled EMT6 spheroid-derived single cells and inert microspheres in unlabeled, intact EMT6 multicellular spheroids has been examined. CB inhibited adhesion and internal migration, whereas nocodazole did not stop adhesion but did prevent later internalization. Treatment of labeled cells with 5,15 and 25 Gy 250 kV X-rays before adherence did not effect their adherence or later internalization. The same radiation treatments administered to the spheroids either immediately before or after the introduction of unirradiated single cells did not affect adherence, but the depths reached by labeled cells and microspheres were reduced largely because of the consequent reduction in spheroid growth. Microsphere size (9, 15, or 25 .mu.m) and surface charge (negative, or nonionic) had minimal, if any, effect on the adherence and internalization of these particles.
