AbstractgCap39 is a newly identified member of the Ca2+‐ and polyphosphoinositidemodulated gelsolin family of actin binding proteins which is different from gelsolin in several important respects: it caps filament ends, it does not sever filaments, it binds reversibly to actin, it is phosphorylated in vivo, and it is also present in the nucleus. gCap39 and gelsolin coexist in a variety of cells. To better understand the roles of gCap39 and gelsolin, we have compared their relative amounts and intracellular distributions. We found that gCap39 is very abundant in macrophages (accounting for 0.6% of total macrophage proteins), and is present in 12‐fold molar excess to gelsolin. Both proteins are highly induced during differentiation of the promyelocytic leukemia cell line into macrophages. gCap39 is less abundant in fibroblasts (0.04% total proteins) and is present in equal molar ratio to gelsolin. The two proteins are colocalized in the cytoplasm, but gCap39 is also found in the nucleus while gelsolin is not. Nuclear gCap39 redistributes throughout the cytoplasm during mitosis and is excluded from regions containing chromosomes. Our results demonstrate that gCap39 is a nuclear and cytoplasmic protein which has unique as well as common functions compared with gelsolin. © 1993 Wiley‐Liss, Inc.