Dissociation of apolipoprotein A-I from pig and steer high density lipoproteins (HDL) deficient in apoA-II was determined by exposing native HDL fractions to 6 M guanidine hydrochloride (Gdn-HCl) at 37 degrees C for periods from 5 min to 18 h. Bovine high density lipoprotein (HDL-B) was isolated at d 1.063--1.100 g/ml while porcine high density lipoprotein (HDL-P) was isolated at d 1.125--1.21 g/ml. Incubation for 5 min with Gdn-HCl resulted in a 45 and 3% loss of apo-A-I from HDL-P and HDL-B, respectively. Exposure to the denaturant for 3 h resulted in a 75% loss of apoA-I from HDL-P and a 30% loss from HDL-B. Analytic ultracentrifugation, patterns paralleled the degree of apoA-I dissociation from each HDL species. The initial flotation peak for HDL-P shifted from F degrees 1.20 2.68 to F degrees 1.20 10.75 after 3 h exposure while HDL-B showed only a small shift from F degrees 1.20 8.30 to F degrees 1.20 8.96 after 3 h exposure. HDL-P particle diameter increased 25% after 5 min of Gdn-HCl treatment and large, flattened structures predominated after 3 h. There was no changes in the size of HDL-B after 5 min exposure and only 16% increase in particle diameter after 3 h. The difference in behavior of HDL-B and HDL-P to Gdn-HCl exposure is discussed in terms of differences in apolipoprotein A-I amino acid composition, interaction of apolipoprotein A-I with phospholipids and the possible involvement of the cholesteryl ester core.