The phenothiazine, trifluoperazine, and the mitogenic lectins, phytohemagglutinin (PHA) and Concanavalin A (Con A), were tested for their effects on human lymphocyte plasma membrane Ca-activated Mg-ATPase and ATP-dependent calcium uptake. Trifluoperazine completely inhibited Ca-uptake when present from the start of the assay at concentrations of 100 microM or more. When added during measurement of calcium uptake, trifluoperazine reduced the rate of vesicular calcium accumulation but was unlike the calcium ionophore, A23187, which caused a rapid release of accumulated calcium from the vesicles. Trifluoperazine also inhibited membrane vesicle Ca-ATPase activity, but this inhibition was non-specific since the Mg-ATPase and Na,K-ATPase activities were inhibited to similar extents at the same concentration of the phenothiazine. In contrast, concentrations of PHA and Con A, which are mitogenic for lymphocytes, did not cause any change in Ca-uptake when added to suspensions of membrane vesicles. Con A had no effect and PHA had a weak inhibitory effect on Ca-ATPase activity.