Abstract At least six species of low molecular weight nuclear RNA (LMN RNA) can be identified in non-dividing human lymphocytes at early culture times. These were designated A (4S) B, B′, C (5S), D and E. After treatment with phytohemagglutinin (PHA) for approx. 48 h, an additional species F, was detected in the nucleus. Furthermore, the proportion of label in species B and B′ was markedly reduced. These results suggest that specific alterations in the metabolism of certain LMN RNAs may occur during the PHA-induced transformation of human lymphocytes.