Increasing evidence indicates that cancer development requires changes both in the precancerous cells and in their microenvironment. To study one aspect of the microenvironmental control, we departed from Michael Stoker's observation (Stroker et al, J Cell Sci 1966;1:297-310) that normal fibroblasts can inhibit the growth of admixed cancer cells (neighbour suppression). We have developed a high-throughput microscopy and image analysis system permitting the examination of live mixed cell cultures growing on 384-well plates, at the single cell level and over time. We have tested the effect of 107 samples of low passage number (<5) primary human fibroblasts from pediatric and adult donors, on the growth of six human tumor cell lines. Three of the lines were derived from prostate carcinomas, two from lung carcinomas and one was an EBV transformed lymphoblastoid line. Labeled tumor cells were grown in the presence of unlabeled fibroblasts. The majority of the tested fibroblasts inhibited the proliferation of the tumor cells, compared to the control cultures where labeled tumor cells were co-cultured with unlabeled tumor cells. The proliferation inhibiting effect of the fibroblasts differed depending on their site of origin and the age of the donor. Inhibition required direct cell contact. Mouse 3T3 fibroblasts inhibited the growth of SV40-transformed 3T3 cells and human tumor cells, showing that the inhibitory effect could prevail across the species barrier. Our high-throughput system allows the quantitative analysis of the inhibitory effect of fibroblasts on the population level and the exploration of differences depending on the source of the normal cells.