We have studied the degree to which fluorescent Ca2+ indicator dyes, and green fluorescent protein and its variants, can be used together. We find that the most commonly used fluorescent protein, enhanced green fluorescent protein (EGFP), seriously contaminates fura 2 signals. We suggest two alternative combinations for which there is no detectable contamination of the Ca2+ indicator signal by the fluorescent protein. Blue fluorescent protein can be used with the Ca2+ indicator Fura Red; EGFP can be used with the Ca2+ indicator X-Rhod 1. The use of these combinations will permit the accurate measurement of Ca2+ signals in cells transfected with fluorescent proteins.

Abbreviations used: AM, acetoxymethyl; BFP, blue fluorescent protein; EGFP, enhanced green fluorescent protein; GFP, green fluorescent protein; RBL, rat basophil leukaemia.

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