Heparin and endogenous heparinoids inhibit the proliferation of smooth muscle cells, including renal mesangial cells; multiple effects on signaling pathways are well established, including effects on PKC, Erk, and CaMK-II. Many studies have used heparin at concentrations of 100 microg/ml or higher, whereas endogenous concentrations of heparinoids are much lower. Here we report the effects of low-concentration (1 microg/ml) heparin on activation of several kinases and subsequent induction of the c-fos gene in mesangial cells in response to the calcium ionophore, ionomycin, in the absence of serum factors. Ionomycin rapidly increases the phosphorylation of CaMK-II (by 30 s), and subsequently of the EGF receptor (EGFR), c-Src, and Erk 1/2. Low-dose heparin suppresses the ionomycin-dependent phosphorylation of EGFR, c-Src, and Erk 1/2, but not of CaMK-II, whereas inhibition of activated CaMK-II reduces phosphorylation of EGFR, c-Src, and Erk. Our data support a mechanism whereby heparin acts at the cell surface to suppress downstream targets of CaMK-II, including EGFR, leading in turn to a decrease in Erk- (but not c-Src-) dependent induction of c-fos.