Gelatin microspheres (ms) and gelatin/BSA (bovine serum albumin) or gelatin/alginate ms were prepared by encapsulating fluorescein isothiocyanate (FITC) labeled dextran or interferon alpha (IFN-alpha). Ms were obtained by an emulsion-solvent-extraction method. Gelatin and gelatin/BSA ms were obtained by treating water-in-oil (W/O) emulsions with iso-propyl alcohol. Gelatin/alginate ms having different composition (25/1, 20/1, 15/1, 10/1 and 5/1) were obtained by treating a W/O emulsion composed of gelatin and sodium alginate with 0.5 M calcium chloride solution. The average diameters of all the prepared ms were approximately 300 microns. The FITC-dextran loading efficiencies were 96.5 +/- 0.6% for gelatin ms (#1), 97.3 +/- 2.2% for gelatin/BSA ms (#2) and 68.7 +/- 2.2%, 55.0 +/- 3.9%, 47.5 +/- 3.3%, 44.4 +/- 1.2%, 27.1 +/- 2.2% for gelatin/alginate ms (#3-#7). The IFN-alpha loading efficiencies were 10.8 +/- 0.5% for gelatin/BSA ms (#8) and 22.5 +/- 1.8%, 17.6 +/- 0.9% and 14.5 +/- 0.5% for gelatin/alginate ms (#9, #10 and #11). In vitro release studies with ms containing FITC-dextran showed that the release rate of FITC-dextran from the ms decreased by the modification of gelatin ms with BSA or sodium alginate, although the effect of BSA addition to gelatin ms did not elucidate satisfactory sustained-release characteristics of FITC-dextran after subcutaneous (s.c.) injection to rats. By decreasing the formulated ratio of gelatin/alginate from 25/1 to 5/1, the mean T50%, the time when the half amount of FITC-dextran contained was released from the ms, increased from 1.7 +/- 0.1 to 13.8 +/- 3.6 h and three ms preparations (#4, #5 and #6) showed sustained-release characteristics on the serum FITC-dextran concentration-time profiles. Based on these results, three types of ms containing IFN-alpha were prepared and in vivo pharmacokinetic studies were performed in rats, where the dose of IFN-alpha was 2 x 10(4) IU/rat. By the addition of alginate to gelatin, the release rate of IFN-alpha was decreased and the serum IFN-alpha concentration-time profiles showed better sustained-release characteristics of IFN-alpha from #10 ms than the other IFN-alpha ms (#8, #9 and #11) after s.c. injection to rats.