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In a previous paper in this journal [Djondjurov, L., Ivanova, E. and Tsanev, R. (1979) Eur. J. Biochem. 97, 133–139], we showed that a nuclear fraction released from chromatin under a mild nuclease digestion contained an increased amount of hnRNA and the bulk of nonhistone proteins with a high metabolic rate. The present investigation has revealed that the nonhistone proteins of this fraction could be divided into three distinct metabolic groups. The first group consists of proteins with a fast turnover rate (mean half-life 30 min) which migrate into chromatin immediately after their synthesis. These proteins are predominantly acid-soluble and have relatively high molecular weights. The second group includes proteins which migrate to the nucleus more slowly and metabolize with a moderate turnover rate (mean half-life 5 h). The third group contains proteins with a more conservative metabolic behaviour. In experiments with actinomycin D it was found that the bulk of the nonhistone proteins of this fraction are not real components of the chromatin but belong to the protein moiety of heterogeneous nuclear ribonucleoprotein particles associated with chromatin. [ABSTRACT FROM AUTHOR]