Four established human breast tumour cell lines with different biologic properties were selected for study and requirements for their clonogenic growth in semisolid cultures were identified. The conventional conditions were modified by factors that enhanced colony formation of 3 or more of these cell lines. The modified culture conditions were then applied to the growth in agar of primary breast tumours. A 5-fold improvement in plating efficiency was observed when cultures of 105 primary tumours grown under these modified conditions were compared to those of 52 tumours grown earlier under conventional conditions, and a 4-fold improvement resulted from the addition of hormones and conditioned medidum in 26 tumours cultured simultaneously under both conditions. The biologic relevance of these clonogens recovered in vitro was substantiated by a 70% concordance of in vitro and in vivo tumour sensitivity to anticancer drugs.