The effects of drugs that cause neutropenia upon colony formation by bone marrow cells in semi-solid agar.

1. The effect of drugs that cause neutropenia has been tested upon normal bone marrow colony formation in semi-solid agar. 2. Thiouracil, methimazole and phenylbutazone do not cause inhibition whereas sodium aurothiomalate, 6-mercaptopurine and chloramphenicol cause doserelated inhibition of colony formation at or below drug concentrations found in serum during therapy. Chloramphenicol also inhibits mouse bone marrow, L cells and Hela cells in agar culture. 3. Dose-response curves have been analysed by the probit method and found to be reproducible.