Distribution of age and sex among patients with Clostridium difficile enterocolitis shows an increased ratio female: male (3:1) in age group 20 to 40 years and a corresponding 2:1 ratio in patients over 70 years of age, the latter group constituting 45% of 505 patients investigated. Being the only laboratory in Sweden performing cytotoxin-assay from cases of C. difficile enterocolitis during 1979-1981, we have observed that the frequencies with which clindamycin/lincomycin are associated with C. difficile enterocolitis have decreased from 30% (1979) to 9% (1981) while the role of cephalosporins has increased from 18% to 29%. Corresponding figures for the penicillins are 31% and 47% for the respective years. Against a background of sales statistics of these antibiotics in Sweden the risk of contracting C. difficile enterocolitis is appreciated for clindamycin/lincomycin and the cephalosporins in comparison to that of the penicillins. These calculations yielded a 63 times greater risk for clindamycin/lincomycin and a 36 times greater risk for the cephalosporins compared to that of penicillins. The susceptibility patterns of 96 clinical isolates of C. difficile are given showing an increased resistance to ceftriaxon, cephalotin, cefoperazone, cefoxitin and moxalactam, as compared to the susceptibility to other antibiotics. The cytotoxin and the enterotoxin produced by C. difficile are described. In our hands the enterotoxin causes hemorrhagic fluid accumulation and gross mucosal damage to rabbit ileal mucosa while the cytotoxin causes only mild inflammatory changes. It is pointed out that the assay demonstrating toxin in cases of C. difficile enterocolitis currently used in practise only measures the presence of cytotoxin and that a routine method for demonstration of enterotoxin is lacking to date.