Abstract Four alginate (irreversible hydrocolloid) impression materials certified as meeting A.D.A. specification 18 for hydrocolloid impression materials were tested for compatibility with five dental stones (20 tests). Five samples of each alginate-stone combination were made and evaluated (100 casts). The method of determining compatibility was a modification of the one given in A.D.A. specification 18 for dental hydrocolloid impression material. The principal changes from the test for compatibility with gypsum given in specification 18 are as follows: 1. The 0.025 mm. wide line produced by the stainless steel test block was used instead of the 0.075 mm. wide line for the evaluation. 2. Each test sample was scored 1, 2, 3, or 4 according to the quality of reproduction of the 0.025 mm. wide line. 3. Evaluation and scoring were accomplished under magnification (x10) using reproducible low-angle illumination. 4. Alginate impression materials and stones used in the study were mixed with a mechanical spatulator under reduced atmospheric pressure. The test casts were scored by four dentists on three successive days. An analysis of variance procedure was applied to the sums of the three replicates for each dentist. Di-Keen appeared to be more compatible with the impression materials tested than were the other stones; whereas, Jeltrate generally appeared to be more compatible with the dental stones tested than any of the other impression materials. The Di-Keen-Jeltrate combination appeared to be the most compatible combination, but it was not statistically better than the Di-Keen-D-P and Vel-Mix-Jeltrate combinations.