Five tumor cell lines that originated from a single mouse mammary adenocarcinoma, normal mammary tissue, a preneoplastic alveolar nodule line, and a tumor developing spontaneously from that preneoplastic line were used to study the different three-dimensional growth patterns that mammary tissues produce in collagen gel. We describe five different outgrowth morphologies, one of which may represent normal stromal tissue and infiltrating cells. One type was produced both by tumors and by normal mammary gland tissue in an age-related fashion, i.e. more outgrowths of this type were produced by mammary tissue from older mice. The outgrowth patterns for the tumor cell lines were not related to morphology in monolayer culture. Certain of the tumor lines, including one variant-producing line, produced multiple outgrowth patterns. Our results indicate that this cultivation technique may be a useful method for studying the heterogeneity of mammary tumors and may facilitate the isolation of mammary tumor subpopulations.