Updated project metadata. Cancer is driven by genetic aberrations in the tumor cells and fundamental changes in the tumor microenvironment (TME). These changes offer potential targets for novel therapeutics, yet lack of in vitro 3D models recapitulating this complex microenvironment impedes such progress. Here, we generated tumor-stroma spheroids reflecting the in vivo TME of early and advanced breast tumor, using a high throughput microfluidic system. The spheroids, composed of tumor cells, fibroblasts and activated immune cells, were generated and cultivated on-chip in alginate (Alg) or alginate-alginate sulfate (Alg/Alg-S) hydrogels.