The human G0/G1 switch (G0S) gene, G0S24, and its rodent immediate-early homolog (TIS11, TTP, NUP475) are part of a mammalian gene family whose members encode CCCH zinc finger domains and domains similar to part of the large subunit of RNA polymerase II and to the Mei2 regulator of G1 arrest in fission yeast. We compared the RNA expression of G0S24 with that of other G0S genes in cultured blood mononuclear cells and examined the levels of various RNA processing intermediates. Freshly isolated cells contained high levels of several G0S RNAs, which declined by 24 h, suggesting transient spontaneous stimulation during cell purification (Heximer et al., 1996). However, in cells preincubated for 24 h, G0S24 RNA levels remained much higher than those of other G0S genes (107+/-42 x 10(6) molecules/microg of RNA); stimulation with lectin (Con-A) further increased G0S24 RNA, much of which remained nuclear. Like those of FOS/G0S7, EGR1/G0S30 and of the gene encoding the regulator of G protein signalling 1 (RGS1), G0S24 RNA levels increased more in response to a protein kinase C activator than to a calcium ionophore, whereas the opposite held for FOSB/G0S3 and RGS2/G0S8. With appropriate PCR primer pairs, we showed a G0S24 RNA processing intermediate, which crossed the exon-1/intron boundary, and nonpolyadenylated nuclear RNA extending into the 3' flank, where there is a second CpG island. The concentration of the latter intermediate (1.2+/-0.2 x 10(6) molecules/microg of RNA), which increased transiently on cell stimulation, did not account for all G0S24 nuclear RNA. The levels of G0S24 RNA and both intermediates were increased by the protein synthesis inhibitor cycloheximide, consistent with regulation by a labile repressor.