AbstractProcaine penicillin [penicillin (procaine salt)] added to a good practical ration improved the growth of chicks in a laboratory used for poultry for ten years. Chicks from the same batch in two other laboratories, where birds had not been kept before, grew equally well on the ration with and without penicillin, and growth was the same as that on the penicillinsupplemented diet in the usual chick laboratory.The growth depression in the absence of dietary penicillin was not due to differences in management or to recognizable disease. It is suggested that it is due to an ‘infection’ hitherto undescribed and shown to be transmissible and counteracted by penicil