Serious delinquencies on subprime mortgages rose to their highest level in more than a year during the fourth quarter of 2000. According to the San Francisco-based Mortgage Information Corp., some 6.1 percent of outstanding subprime loans were 90 days past due or in foreclosure at the end of December. That was up markedly from September’s 5.53 percent serious delinquency rate. Overall, serious delinquencies on subprime paper jumped 21 percent from the previous December, MIC reported.