The concentration of beta-2-microglobulin (beta 2-m) and of post gamma globulin (P gamma G) was examined in serum and cerebrospinal fluid from children with acute lymphatic leukemia (ALL) and non-Hodgkin's lymphoma (NHL). Data were analysed in order to determine whether concentration of beta 2-m or P gamma G during remission would be of value in predicting relapse or eventual outcome. Mean serum concentration of beta 2-m was similar in good and poor prognosis patients with ALL in remission and was not significantly altered in CNS or marrow relapse. Mean CSF concentration in NHL was also similar in both prognostic groups, and in poor prognosis patients was not significantly altered in relapse. The same pattern was seen when P gamma G was measured in CSF (serum concentration of this protein being too low for accurate determination). High within patient variability of levels of beta 2-m and P gamma G appeared to relate to chemotherapy rather than the disease process. Concentration of P gamma G was persistently raised in three children with brain damage of differing etiologies. Levels of two other low molecular weight proteins, retinol binding protein and alpha 1-microglobulin, were also determined in order to establish that beta 2-m and P gamma G concentration was not influenced by alteration in permeability of the blood-brain barrier. The beta 2-m and P gamma G concentration, although higher than reported in healthy children [5] does not appear to be of value as a prognostic indicator in ALL and NHL in children.