PIP: 14 6-substituted-Delta6-16-methylene-17alpha-hydroxy-4,6-prenadiene-3, 20-dione 17-acetate derivatives were analyzed by multiparameter regression techniques for quantitative structure-activity relationships. The parameters were published values for hydrophobic bonding power (pi), inductive effect (F), resonance effect (R), and molar refraction of the substituent (MR); the activity was progestational activity by Clauberg test. Least squares analysis showed that pi gave the best fit with single parameters. With 2 parameters, pi and F accounted for 69% of the variance. F and pi squared gave the best 3-term results, suggesting that pi values up to .50 permit the maximum progestational activity, but perhaps larger substituents decrease activity. The 6-methyl derivative was an exception, having 4 standard deviations' higher activity than computed by the 3-term equation, pointing to unusual metabolism.