Previous platelet studies have shown that calcium plays important roles in stimulus-secretion coupling, aggregation, and other membrane-associated functions. In addition, lanthanum induces platelet aggregation and the platelet release reaction and also influences platelet responsiveness to various stimuli. The spin-label results presented here suggest that one mechanism through which calcium and lanthanum mediate their effects on platelet functions may be by decreasing the lipid fluidity of the surface membrane. The structure of platelet membrane lipids was examined with the spin-label method. Washed human platelets were labeled with the 5-, 12- and 16-nitroxide stearic acid spin probes. Order parameters which measure the fluidity of the lipid environment of the incorporated probe may be calculated from the electron spin resonance (ESR) spectra of 5-nitroxide stearate [I(12,3)]-labeled cells. Evidence is presented which indicates that these spectra principally reflect properties of the platelet surface membrane lipids. The membrane fluidity increased with temperature for the range 17 to 37 degrees C. Either calcium or lanthanum additions to intact cells increased the rigidity of the platelet membranes at 37 degrees C, although the La3+ effect was larger and occurred at lower concentrations than that of Ca2+. For example, addition of 1 mM La3+ or 4 mM Ca2+ increased the order parameter of I(12,3)-labeled platelets by 4.3 +/- 1.7% or 2.1 +/- 0.5%. Preliminary studies conducted on purified platelet plasma membranes labeled with I(12,3) indicated that 1 mM LaCl3 or 4 mM CaCl2 additions similarly decreased the lipid fluidity at 37 degrees C. The above cation-induced effects on the fluidity of whole platelets were reversed by the use of the divalent cation-chelating agent ethylene glycol-bis-(beta-aminoethyl ether)-N,N'-tetra-acetic acid (EGTA). Lastly, lanthanum (0.2-1 mM) caused rapid aggregation of platelets which were suspended in a 50-mM Tris buffer pH 7.4 that did not contain adenosine.