A new class of synthetic biological response modifiers (BRMs) has been produced by combining a highly electrophilic reactant, 2-methyl-2, 5-dihydrofuran (a cyclic acetal of cis-3-acetyl acrolein), with L-ascorbic acid. The parent class of compounds can be referred to as methylfurylbutyrolactones (MFBL), previously termed Nafocare B. This parent molecule is amorphous, has a molecular weight of 252.7, and the chemical name [3,6] cyclohemiketal of 2-(5-methyl-2-furyl)-3-keto-L-butyrolactone. Two crystalline forms were produced by a reaction of the MFBL parent molecule with either succinic anhydride or succinimide, to create MFBL-SA (Nafocare B2) and MFBL-S (Nafocare B3) dimers, respectively. The structure of these compounds has been confirmed by modern methods of analytical chemistry, including x-ray crystallography. All three forms of the MFBLs showed negligible toxicity in single-dose acute LD-50s in mice. Also, the MFBLs did not demonstrate genotoxic activity at 800 mg/kg in the mouse micronucleus assay. The MFBLs are immunostimulatory in assays involving T- and B-lymphocytes, but not in immunoassays on macrophages derived from resident- or thioglycollate-elicited peritoneal exudate cells (PEC). Spleen cells from mice treated 4 days via the intraperitoneal, intravenous, or the oral routes responded significantly over controls to suboptimal stimulatory concentrations of polyclonal mitogens in the lymphocyte stimulation assay. The MFBLs were not mitogenic, since they did not increase DNA synthesis in resting spleen cells from MFBL-treated mice. Antibody production is also amplified by the MFBLs. Mice immunized with sheep erythrocytes, a T-cell-dependent antigen, and treated with MFBLs had a 200-800% increase in the numbers of antibody-producing splenic lymphocytes detected by the Jerne hemolytic plaque assay. Also, mice immunized with soluble bovine serum albumin (BSA), and treated with a MFBL, demonstrated at least a fourfold increase in IgG-specific antibodies to BSA, when compared with controls. To demonstrate effects of MFBLs on macrophages, we used the Fc receptor (FcR) surface marker and superoxide anion assays. Only at the highest in vitro dose of MFBL (16 micrograms/ml) was a significant increase in erythrocyte antibody rosette formation detected, using resident macrophages isolated from PEC. In the superoxide anion release assay neither resident- nor thioglycollate-elicited PECs, obtained from in vivo-treated mice or macrophages treated in vitro, showed increased production of superoxide anion.(ABSTRACT TRUNCATED AT 400 WORDS)