Serum levels of lactoferrin, lysozyme and myeloperoxidase have been established in 31 healthy children. On average, serum lactoferrin was 330 micrograms/1, serum lysozyme 1638 micrograms/1 and serum myeloperoxidase 174 micrograms/1. Serum myeloperoxidase was, on average, significantly higher in children than in adults (p = 0.01), whereas serum lactoferrin and serum lysozyme were equal to those of adults. In a group of infection-prone children (n = 31), both serum lactoferrin and serum myeloperoxidase, but not the serum lysozyme levels, were significantly lower (p less than 0.001 and p = 0.002, respectively) than those of the reference children in spite of normal intracellular contents and even somewhat higher peripheral blood polymorphonuclear counts. Based on the assumption that serum lactoferrin and serum myeloperoxidase reflect turnover and activity of neutrophil granulocytes, the findings could suggest reduction in these respects and could be one contributing factor to the high infection propensity of these children. Serum levels of the three proteins have also been measured in 10 children with suspected or various forms of manifest leukemia. It is suggested that the levels reflect turnover and stage of maturation of the myeloid and monocytic cells and could, therefore, aid in the understanding and diagnosis of these diseases.