A combination of methods was used to study the cell cycle-dependent expression of nuclear matrix proteins of Ehrlich ascites cells: 1. 1. Separation of asynchronous cells growing in vivo into fractions of G1-, S- and G2-phase cells by centrifugal elutriation with less than 10% cross-contamination. 2. 2. Isolation of poly(A +) RNA populations from total cytoplasmic RNA by affinity chromatography on messenger affinity paper (mAP). 3. 3. In vitro translation of poly(A +) RNA from asynchronous and phase synchronous cells. 4. 4. Immunoprecipitation of in vitro synthesized nuclear matrix proteins by a monoclonal antibody with anti-lamin specificity (PKB8) and by a polyspecific anti-nuclear matrix serum (AMS5) followed by analysis of immunoprecipitated materials on SDS-polyacrylamide gels. The results indicate that mRNAs for nuclear matrix-associated proteins including the lamins B and C are either exclusively or at least predominantly present in the cytoplasm of cells in S phase suggesting a high rate of in vivo synthesis of these proteins during S phase. This is consistent with an anticipated biological function of the nuclear matrix which is considered to organize parental and newly synthesized DNA in higher order structures.