Fragmented sarcoplasmic reticulum (FSR) was prepared from the white muscles of the catfish (Amiurus nebulosus). The effect of La3+ on the functional characteristics of FSR was studied. La3+ in a concentration higher than 10(-4) M was found to decrease or arrest Ca2+ accumulation, cholinesterase activity and the activation of ATPase by Ca2+. La3+ added after the elimination of membrane-bound Ca2+ of FSR (1 mM EGTA, pH 7.1) does not substitute Ca2+ in its functions. The cholinesterase activity of FSR solubilized by deoxycholate and purified by gelfiltration is inhibited by La3+ present in a concentration higher than 10(-4) M and simultaneously with inhibition, the absorbance at 280 nm is increased. Ca2+ gives rise to similar changes only in concentrations higher than 10(-2) M.