The clonogenicity in soft agar and the labeling index (LI) which represents the percentage of cells in the DNA synthesis phase, were studied in 59 breast cancer patients and these parameters were related to other known clinicopathological features, namely age, histological grading, estrogen and progesterone receptors and the status of axillary lymph nodes. Out of 59 tumors, 49 could be successfully cloned in soft agar and the mean plating efficiency (PE) was 0.1%. Low grade tumors were more frequently encountered in tumors which did not form colonies (P = 0.025). Cloned tumors had a higher mean LI (P = 0.05). A high PE was associated with low estrogen receptors (ER) (P = 0.03). Clonogenicity was not related to patient age, progesterone receptors (PR) or the status of axillary lymph nodes. These results suggest that a successful in vitro cloning and a high PE are associated with unfavorable prognostic factors in breast cancer.