Infiltration of monocytes into multicellular spheroids of J82 and MelIm. MCTSs were generated using 5 × 10³ suspended cells from exponentially growing tumor cell monolayers and cultured on 1% solid agarose in 96-well plates. After 4 to 5 days, half of the medium was replaced by a monocyte suspension containing 4 × 10⁴ monocytes. MCTS cocultures were harvested 1 day later. Images show 5-μm paraffin sections stained for the leukocyte marker CD45 using a routine peroxidase technique with DAB as chromogen and hematoxylin counterstain to visualize infiltrated monocytes (bar = 100 μm).