We describe effects of ovine colostrinine (proline-rich polypeptide--PRP) isolated from ovine colostrum and nonapeptide fragment of PRP on interferon (IFN) and tumor necrosis factor (TNF) production by murine resident peritoneal cells (RPC). The cells from several mouse strains have been found to produce small amounts of IFN-beta and TNF-alpha constitutively. The colostrinine at concentrations of 1-100 micrograms per one ml of cell suspension containing 1 x 10(6) RPC isolated from BALB/c mice, enhanced the IFN and TNF production by 3-30 folds. Upregulation of TNF and IFN production has been observed in the RPC cultures that produced spontaneously less than 16 units of the cytokines only. Synthetic nonapeptide fragment of the colostrinine (Val-Glu-Ser-Tyr-Val-Pro-Leu-Phe-Pro) at concentration of 1-100 micrograms/ml stimulated TNF synthesis but not IFN production. In 1996 Inglot et al. suggested that the colostrinines may be classified as cytokines produced by the mammary gland of mammals. In this paper we have found that the ovine colostrinine at low concentrations modulate the production of other cytokines (IFN-beta and TNF-alpha) in mouse cells that means that it may function in the cytokine network.