The high-frequency resistance, R HF, or internal resistance, of 45Ah flooded tubular lead-acid battery (LAB) cells was monitored during cycling at constant rates between C/100 and C/10 in order to understand the origin of the R HF variations and to evaluate the feasibility of monitoring the state-of-charge (SOC) with this parameter. It is shown that the R HF variations did not depend on the electrolyte-conductivity variations, as usually indicated in the literature. At a low discharging rate (C/100), R HF increased only at low SOCs, with a high final value, while at a high rate (C/10) R HF increased progressively from the beginning of the discharge, with a low final value. This has been interpreted as the consequence of the shape and size of the PbSO4 crystals in the pores of the active material, which are the result of the continuous competition between crystal nucleation and crystal growth. R HF informs on the structure of the PbSO4 layer, and therefore, depends strongly on the history of the previous cyclings of the cell. Monitoring the SOC from the single value of R HF was found to be impossible since, for different cycling rates, distinct values of SOC may correspond to the same value of R HF. Fluctuations of R HF were also measured at different SOC: they allow detection of gas evolution in the cell and could provide complementary information for estimating the SOC of LABs.