We have previously reported that the human homologue of the Drosophila transient receptor potential protein hTRPC1 is involved in conducting store-operated Ca²⁺ entry (SOCE) in human platelets. An antibody raised against the pore-forming region of hTRPC1 inhibited Ca²⁺ entry evoked by Ca²⁺ store depletion using thapsigargin and that evoked by the physiological agonist thrombin (Rosado et al. 2002). Furthermore, coimmunoprecipitation experiments show that hTRPC1 associates de novo with the type II inositol 1,4,5-trisphosphate receptor when the intracellular Ca²⁺ stores are depleted. We have suggested that this coupling event might underlie the activation of SOCE in platelets. However, others have reported that hTRPC1 is localised in internal membranes in platelets, questioning its role in SOCE (Hassock et al. 2002). Here we have investigated plasma membrane expression of hTRPC1 in human platelets and, using a range of specific antibodies, have probed for the presence of other TRPC proteins in these cells and have investigated their associations. Biotinylation revealed the presence of hTRPC1 in the plasma membrane of resting platelets. Surface expression was not detectibly changed following depletion of the intracellular Ca²⁺ stores using 1 μM thapsigargin together with 50 nM ionomycin or when the cells were stimulated with 1 unit/ml thrombin (n = 4). In addition to hTRPC1, Western blotting demonstrated the presence of hTRPC3, hTRPC4, hTRPC5 and hTRPC6 in platelet lysates (n = 6). hTRPC4 and hTRPC5 coimmunoprecipitated with hTRPC1 whereas hTRPC3 and hTRPC6 did not. However, hTRPC3 did coimmunoprecipitate with hTRPC6 and vice versa. hTRPC4 and hTRPC5 were found to be associated with detergent-resistant platelet membranes, from which they were partially released when the cells were depleted of cholesterol by treatment with methyl-β-cyclodextrin (MBCD) (n = 5), as we have previously shown for hTRPC1 (Brownlow et al. 2004). In contrast, the distributions of hTRPC3 and hTRPC6 between soluble and membrane fractions were not affected by MBCD treatment. These results suggest that hTRPC1, hTRPC4 and hTRPC5 form a heteromultimer that is associated with platelet lipid raft domains. This might mediate SOCE. hTRPC6, which has been suggested to conduct store-independent (non-capacitative) Ca²⁺ entry in platelets, may form another heteromultimer with hTRPC3.