An in vivo-in vitro implantation model has been used to investigate further the early stages of chemically induced s.c. neoplasia in the mouse. Cell cultures of implant-site tissues from control and 3,4-benzpyrene (BP)-treated animals were found to mirror the in vivo tissue reactions occurring at the time of explantation (Westwood et al., 1979). Cells were classified into 6 different types. The most abundant cell type in later control cultures was of a typical fibroblast morphology. However, a suppression of growth of fibroblast-like cells occurred when BP-treated tissues were explanted, and a selection of growth in favour of the large polygonal Type 5 cells was observed. When grown from BP-treated tissues Type 5 cells were found to be capable of growth in a semi-solid agar medium. Quantitative studies showed that cells capable of growth in agar reached a peak about 4 weeks after implantation, followed by a decline in numbers until the formation of tumours. This observation may result from the parameters regulating the development of chemically induced neoplasia in the subcutis.