The frequency of isolation and the number of staphylococci and Gram-negative enteric rods in saliva were estimated in a group of 48 children of both sexes, aged 4-15 who were in the course of treatment of acute lymphoblastic leukemia (ALL). The results were compared with the same parameters in a group of 44 healthy children of the same age. Staphylococci (both coagulase-negative and coagulase-positive) were found in the oral cavities of all healthy children and of 91.6% children with ALL. However, the diseased children harbouring staphylococci revealed a significantly higher average number of these bacteria than healthy children; the respective values were 3.59 and 3.02 log CFU/ml of saliva (p = 0.0148). The average number of staphylococci in saliva was in both groups of children in negative correlation with the number of polymorphonuclear granulocytes in blood. Gram-negative enteric rods were present in the oral cavities of 13.6% of healthy children and 33.3% of children with ALL (p = 0.0005), but the counts of these bacteria were comparable. The negative correlation between the number of these bacteria and the number of polymorphonuclear granulocytes in blood was not significant.