Phage and biochemical typing of Salmonella typhi-murium strains performed in the course of the Salmonella Surveillance Programme were completed with examinations on resistance to antibiotics and R-plasmids. A total of 15 600 strains of human, animal and water origin were tested between 1975 and 1981 and most of the monoresistant strains were found among the animal strains (73.6%-94.9%), while double resistance was the most frequent among the human strains (5.5%-25.5%) and multiresistance occurred in the highest ratio (2.8%-25.6%) among the strains of water origin. Tetracycline resistance was the most frequent in all the three materials. The curves representing the incidence of tetracycline (Tc), chloramphenicol (Cm), streptomycin (Sm), kanamycin (Km) resistance were similar to the curve of multiple resistance and differed from the curve showing the incidence of ampicillin (Ap) resistance. Gentamicin resistance was found only among human strains (in 0.3 and 0.7%), strains resistant to nitrofurantoin and co-trimoxazole occurred among strains of human and water origin in low percentages. The most common antibiotic resistance patterns of the multiple resistant strains were Tc, Cm, Sm, Km, Ap, Su; Tc, Sm, Su; Sm, Km, Ap, Su. Multiple resistant strains belonged in the majority to phage type nt (not typable) and 2b. Out of the examined 512 S. typhi-murium strains resistant to antibiotics, the presence of R-plasmid was demonstrated in 408 strains (i.e. 79.7%). The R-plasmids, derived from strains of human, animal and water origin, of phage type nt, biotype 3 (nt/3) isolated in 1979, were characterized according to the resistance determinants, fi-character, incompatibility-group, phage-inhibition and molecular weight. Two kinds of R-plasmids were carried by three human strains (FI and H, FI and alpha). R-plasmids belonging to Inc P and Inc H were carried by one animal strain. Strains isolated from sewage carried R-plasmids of Inc groups H and I alpha. Out of the examined 15 S. typhi-murium strains of phage type 2b, isolated in 1981, the molecular weight for 7 strains was 66 Md and four belonged to Inc I alpha. The R-plasmids derived from 2b/2 strains, isolated in the same county, were identical according to antibiotic resistance determinants, phage inhibition and molecular weight. The molecular weights of R-plasmids derived from 10 strains out of the examined 30 nt strains were also 66 Md and the four examined plasmids belonged also to Inc I alpha.(ABSTRACT TRUNCATED AT 400 WORDS)