Antiserum to a synthetic peptide that defines a hydrophilic region within the putative c-myb translation product was prepared in the rabbit. In lysates from exponentially growing ML-1, human myeloblastic leukemia cells, the antiserum ("anti-myb") reacted with five proteins of Mr 58,000, 75,000, 85,000, 90,000 and 105,000. Of these, only p75 and a trace of p85 were detected, by immunoblotting, in extracts derived from ML-1 cell nuclei. The proteins p58, p75 and p90 were present in readily detectable amounts only in the relatively immature myeloid cell lines ML-1 and HL-60, whereas in the more mature myeloid cell line THP-1 and in the lymphoid line BALL-1 only traces of these proteins were found. p85 and p105 were detected in lysates from all cell lines tested, including myeloid and lymphoid leukemia cells and mouse 3T3 cells. In lysates from ML-1 cells induced to differentiate to monocyte/macrophages or to granulocytes, the concentrations of p58 and p75 decreased in parallel with the cell population moving to maturity; in completely mature populations these two proteins were no longer detectable. In ML-1 cells arrested in G1 by serum depletion, the amount of p58 and p75 and to a smaller extent that of p90 was decreased, whereas the concentration of p85 and p105 remained unchanged. In nuclei from exponentially growing ML-1 cells, the antiserum or its derived immunoglobulin fraction ("anti-myb IgG") inhibited mRNA transcriptional activity by 30%. DNA synthesis was not affected. In contrast, in nuclei from differentiated ML-1 cells, the mRNA transcriptional activity was not significantly inhibited by anti-myb IgG. Similarly, in nuclei from ML-1 cells arrested largely in G1 by serum depletion for 2 days, mRNA transcriptional activity was inhibited by only 11%. Upon supplementation with serum, the mRNA transcriptional activity inhibitable by anti-myb IgG increased in parallel with the increasing rate of cell growth. The difference in total mRNA transcriptional activity observed in nuclei from cells of different growth rate was accounted for by the difference in transcriptional activity inhibitable by anti-myb IgG.(ABSTRACT TRUNCATED AT 400 WORDS)