Three families are presented in which an infant with null acute lymphoblastic leukemia had a karyotype rearrangement involving a break at 11q23. Peripheral blood was obtained, where possible, from both parents and from the child during periods of remission. The blood was stimulated with phytohemagglutinin and cultured under conditions that enhance expression of heritable folate-sensitive fragile sites. In all individuals studied very low levels of fra(11)(q23.3) were observed. These levels were far below those recorded for expression of the heritable folate-sensitive site fra(11)(q23.3) but are comparable with expression of the common fragile site fra(11)(q23.3) under these conditions.