The death rate among infants born after premature rupture of the fetal membranes in a hospital rose sharply from 29 per 1,000 in 1954 to 144 per 1,000 in 1958. Various factors known to affect neonatal mortality in other situations were eliminated by analysis of the data until it appeared that the rise in death rate had followed the institution of a program of prophylaxis with antibiotics. Among 160 newborn infants who received such prophylaxis from January to May, 1958, 7 deaths occurred among 97 who received chloramphenicol with erythromycin, 5 among 8 who received chloramphenicol, erythromycin, and penicillin, and 5 among 5 infants who received chloramphenicol, erythromycin, penicillin, and streptomycin. Among the infants who died the characteristic syndrome included anorexia, lethargy, respiratory distress, a shock-like state, and a dusky gray appearance. The data were not sufficient to identify a single antibiotic as the sole cause, but no further cases of "gray sickness" occurred after the use of chloramphenicol was discontinued.