Peritoneal exudate cells, obtained from mice injected with thioglycollate medium and cultured in medium containing L cell conditioned medium will proliferate in an exponential fashion for 18 days with a doubling time of 68 hr. After a 2 hr pulse of tritiated thymidine, labeled adherent cells increased to a maximum of 22 to 34% during the 1st and 2nd wk of culture. Increasing the cell concentration from 2x103 to 2x105 cells/culture reduced exponential growth to 10 days and the doubling time was increased to 81.6 hr. Under these culture conditions, peritoneal exudate cells were shown to form colonies on the surface of culture dishes when plated at low density. The cells within the colony were shown to be macrophages using yeast and antibody coated sheep erythrocytes as a test for phagocytic function. The plating efficiency ranged between 4 and 12% of the total viable cells plated and it appears that the colonies arose from a single precursor cell. The adherent cell population contains the colony forming precursors. These precursors can be stimulated to form colonies for at least 2 wk by the addition of conditioned medium to cultures at various times after plating. While very few colony forming cells could be demonstrated in the unstimulated peritoneal lavage, their numbers begin to increase in the exudate 4 hr after injection of thioglycollate medium and reach a maximum by day 3 and then decrease. Isolated colonies may be useful in studying the function of macrophages.