Micellar aggregation numbers, n, have been obtained for sodium dodecyl sulfate (SDS) micelles in cyclodextrin (CD) solutions through static fluorescence quenching. For alpha-cyclodextrin, beta-cyclodextrin, (hydroxylpropyl)-beta-cyclodextrin, maltosyl-beta-cyclodextrin, and gamma-cyclodextrin solutions, n was found to be essentially unchanged from the 59 +/- 5 value found in sodium dodecyl sulfate solutions without cyclodextrin present, even though the critical micelle concentrations as measured conductometrically had increased. The increases in critical micelle concentrations were consonant, with 1:1 CD/SDS complexation predominating over other complexes in all CD systems except for alpha-CD solutions, where the 2:1 alpha-CD/SDS complex prevailed. The association constants of the fluorescence quencher, 9-methylanthracene, with SDS and with each of the pure cyclodextrins were determined to ensure preferred binding of the quencher with SDS micelles rather than with CD. The substituted cyclodextrins, dimethyl-beta-CD and trimethyl-beta-CD, were not amenable to the fluorescence quenching method. The implications of these findings to drug delivery are examined.